Audio Adaptation

Where things are adapted to audio only media such as Radio, Audiobooks, or Audio Plays. Some are full adaptations, usually from Literature, Comic Books, or Manga, giving the then silent characters voices. Others are side-stories for the franchise released in an immerse world of sound, a favorite path for animated fare since, after all, they already have the voice actors and sound effects.

This is especially common in Japan. Since they're pretty cheap to produce (you just need the actors, some sound effects, and mixing equipment to make them), a popular work, be it Manga, Anime, a Video Game or what have you will often get several "Drama CDs" and whatnot as spinoffs, pseudo-sequels, prequels and interquels to fill in the world and characters. Since the going rate for the actors isn't too high (and don't require many takes, etc) and for better franchises the actors can like doing them, they aren't too expensive to produce and thus can be sold fairly cheap at retail, meaning you can produce a fair number of them and still make a good profit.

And, of course, their all-audio nature makes exporting them a goddamned nightmare. Drama CDs are, unquestionably, the #1 source of All There in the Manual problems for exported Japanese products, because the form is very uncommon in America, there's no real place to sell them, the actors have to charge more if they're union which drives costs up, etc etc... so as a result, these basically never leave Japan, which can end up being hugely problematic for fans overseas as they end up missing part of the story (and it's a problem for any producers who care about exporting, since they know it's all but impossible for overseas fans to get that part of the story).

Contrast with Sound to Screen Adaptation.

Anime & Manga

 * Lots of manga and anime have audio dramas, from Nadia: The Secret of Blue Water to Count Cain to Gankutsuou. Then there's the "Nyoron Churuya" audio drama. Usually, these only exist in Japanese.
 * A great number of Manga and Light Novels have Drama CDs released before, during and after their animated adaptations. Sometimes the voice actors between the Drama CD version and the anime version can change. Many a Visual Novel also has a Drama CD included, usually as a bonus. This isn't much of a stretch either, since the line between visual novel and audio adaptation is thin.
 * The Sound Stages of Magical Girl Lyrical Nanoha have been an integral part of the franchise from the start, giving additional side-adventures, back-stories, and setting information that would get referenced in the main part of the series, with certain tidbits in the anime only becoming clear if you've listened to these. The biggest one of these is Striker S Sound Stage X, an entire Story Arc set three years after the third season.
 * The Slayers has several set between anime seasons:
 * Slayers EX (Extra) and Slayers N>EX: Set after the first season and second seasons, there are four stories based off of the prequel Slayers Special novels, but they implement Lina's allies instead of Naga. Naga does appear in N>EX, though.
 * The Return of Slayers EX: Five original stories set after seasons 2 and 3, including
 * Slayers Nextra: Set after the second season, a full-scale adventure, unlike the others.
 * A prologue and epilogue for the Slayers Premium Non Serial Movie; the prologue creates an issue in continuity by setting two seasons five years apart, which supposedly isn't true in-universe.
 * Two dramas based off of the fourth and fifth seasons of the anime; there is an epilogue story for the one based on Evolution-R.
 * A Crossover story starring the Slayers cast and the Sorcerous Stabber Orphen cast.
 * Mahou Sensei Negima and Negima!? had some drama CDs, which if nothing else, are notable for actually referencing the fandom's popular "KonoSetsu" Portmanteau Couple Name in one of the tracks.
 * A more recent one is the Ala Rubra Drama CD. Specifically the following:
 * Gundam Wing had a radio play called Blind Target, complete with its own theme songs, which was later adapted into a manga (causing a form of Adaptation Displacement in America).
 * Both the manga and anime versions of Chrono Crusade spawned drama CDs. The anime version is particularly notable for being a High School AU and allowing the voice actors from the anime to purposefully make fun of their characters by being as hammy as possible.
 * A really notorious example is Getter Robo Armageddon, which was the animation sequel... to a popular, long-running radio drama based on the Getter franchise that 99% of anime fans had never even heard of when Armageddon first made its way to America. Fans end up missing a fair bit of backstory as a result.
 * Gundam Wing had a radio play called Blind Target, complete with its own theme songs, which was later adapted into a manga (causing a form of Adaptation Displacement in America).
 * Both the manga and anime versions of Chrono Crusade spawned drama CDs. The anime version is particularly notable for being a High School AU and allowing the voice actors from the anime to purposefully make fun of their characters by being as hammy as possible.
 * A really notorious example is Getter Robo Armageddon, which was the animation sequel... to a popular, long-running radio drama based on the Getter franchise that 99% of anime fans had never even heard of when Armageddon first made its way to America. Fans end up missing a fair bit of backstory as a result.
 * Both the manga and anime versions of Chrono Crusade spawned drama CDs. The anime version is particularly notable for being a High School AU and allowing the voice actors from the anime to purposefully make fun of their characters by being as hammy as possible.
 * A really notorious example is Getter Robo Armageddon, which was the animation sequel... to a popular, long-running radio drama based on the Getter franchise that 99% of anime fans had never even heard of when Armageddon first made its way to America. Fans end up missing a fair bit of backstory as a result.

Comic Books

 * Superman has been adapted to radio form many times. The first one gave birth to the popular "It's a bird! It's a plane!" opening, and was responsible for creating kryptonite as well. It was also possibly responsible for killing the KKK.
 * Batman likewise has had a few stints, albeit brief, Knightfall being notably adapted by The BBC.
 * Graphic Audio has since joined with DC to bring a number of the company's properties into "Long Car Ride" radio versions, including both original stories, and adaptations of many a Crisis Crossover. The Infinite Crisis one is particularly good.

Film

 * All three original Star Wars movies had radio play adaptations broadcast by NPR: Star Wars in 1981, The Empire Strikes Back in 1983, and Return of the Jedi in 1996. The first one is very famous for including a whole lot of stuff that ended up on the cutting room floor of the original movie, including early scenes on Alderaan with King Organa, a long chat between Biggs Darklighter and Luke that established both their characters, and a greatly expanded interrogation scene between Vader and Leia.
 * As have the following comic book arcs: Tales of the Jedi Knights of the Old Republic, TOTJ Freedon Nadd Rebellion, TOTJ Dark Lords of the Sith, Dark Empire I, II and Empire's End, Crimson Empire, and the three Dark Forces graphic novels. Oddly enough, they never finished them by releasing the two more Tales of the Jedi arcs featuring Ulic Qel-Droma as well as making Crimson Empire II. Dark Empire has some script changes.
 * The Radio show The Lux Radio Theater was a long-running program adapting hit (and some lesser) movies to radio drama form. It was produced and hosted by Cecil B De Mille.
 * The Lux adaptation of Its a Wonderful Life was recently remade, with Bill Pullman as George Bailey.

Literature

 * BBC Radio has produced lots of adaptations of books, many of them starring well-known actors like Christopher Lee and David Warner.
 * The Lord of the Rings, with full cast and sound effects. Starred Ian Holm as Frodo, interestingly (Holm played Bilbo in the films); also, Peter Woodthorpe's Gollum had a pretty clear influence on Andy Serkis'.
 * The Hobbit, with full cast, sound effects, and original material (such as Bilbo talking back to the narrator)
 * All of the Sherlock Holmes stories, with full cast and sound effects
 * The Discworld novels Guards! Guards!, The Amazing Maurice and His Educated Rodents, and Night Watch adapted for radio, with full cast and sound effects
 * A number of Robert Rankin novels, including Brightonomicon and the Brentford Trilogy, with full cast and sound effects
 * Mary Shelley's Frankenstein, full cast, sound effects
 * Ghost stories by a variety of authors, full casts, sound effects
 * A number of Tintin's adventures. In these adaptations, Snowy takes on the role of extraneous narration and comic relief that can't be accomplished through dialogue and would have been accomplished through artwork in the original (and is voiced by Andrew Sachs).
 * Philip Pullman's His Dark Materials trilogy - although there are actually two adaptations of this one: a radio adaptation and a full-cast unabridged recording, narrated by Pullman. Both were produced by the BBC.
 * The Foundation Trilogy, full cast, sound effects
 * War and Peace, full cast, sound effects
 * Adrian Mole. In fact, original Adrian Mole monologues were written for Pirate Radio 4, a teenage magazine show that also included Doctor Who: Slipback (below). These were recursively adapted into "Adrian Mole at the BBC" in True Confessions of Adrian Albert Mole.
 * American publishers don't do this as often, but there have been full-cast recordings of:
 * Robert Heinlein's Have Space Suit, Will Travel
 * Many of HP Lovecraft's novels and stories
 * Star Trek novels, narrated by George Takei and guest starring Leonard Nimoy and William Shatner.
 * Long before the Star Trek audiobooks, audio plays featuring the original cast were produced in LP format.
 * Many Star Wars novels as well, including Matt Stover's greatly respected novelization of Revenge of the Sith. Many of these audio adaptations even include sound effects and music from John Williams's scores.
 * Quite a few Isaac Asimov stories
 * Many of Edgar Allan Poe's stories
 * World War Z was adapted with a full cast
 * Stephen King is a big audiobook fan. The audiobook release of his massive Nightmares and Dreamscapes collection starred different narrators for every story, including such luminaries as Grace Slick, Yeardley Smith (yes, that Yeardley Smith. If you think King is creepy on the page, wait until you hear Lisa Simpson narrating it!), Joe Mantegna, and Tim Curry.
 * Even though the first two Hitchhiker's Guide books were originally radio plays (and then adapted to other media), the last three books were books first and later adapted for radio.
 * The Shadow counts, even though the magazine was inspired by the "narrator" of a mystery radio show. The magazine started in 1931, the radio adaptation in 1937.
 * An unusual case is Kenneth Williams' reading of Nikolai Gogol's Diary of a Madman. This was originally recorded as the narration for an animated adaptation, but the animation was never completed and the recording was eventually broadcast as a radio monologue by The BBC.
 * Ulver adapted William Blake's The Marriage of Heaven and Hell into a two-disc album, using the poem as the lyrics in it's entirety.
 * Focus on the Family (responsible for Adventures in Odyssey) has a radio drama arm, appropriately titled Focus on the Family Radio Theatre. Among other things, they have produced:
 * Les Misérables, starring Brian Blessed
 * Silas Marner
 * The complete Narnia series
 * A Christmas Carol
 * The Screwtape Letters
 * Anne of Green Gables

Live Action TV

 * Doctor Who has a lot of these.
 * The BBC have produced a few radio adaptations of serials such as "Genesis of the Daleks".
 * Official BBC-produced original radio plays have been produced as far back as 1986's "Slipback".
 * Officially licensed audio plays by Big Finish, featuring the continuing adventures of Doctors Five, Six, Seven, and Eight (and later Four), now number in the hundreds. Quality is generally considered very high, especially for Colin Baker's Sixth Doctor.
 * An outfit called BBV produced audio plays and series of varying officialness during the Eighties and Nineties: one starring Colin Baker and Nicola Bryant as "The Stranger" and "Miss Brown"; one starring Lalla Ward and John Leeson as "The Mistress" and "K-9"; and one starring Sylvester McCoy and Sophie Aldred as "The Professor" and "Ace". The first two were official, but the last wasn't--and the BBC stepped in to force some more filing-off of serial numbers.
 * BBC Audio has produced several audiobooks featuring the Tenth and Eleventh Doctors. They also created three series of Fourth Doctor audio dramas, starring Tom Baker and Richard Franklin (reprising his role as Third Doctor "companion" Mike Yates).
 * BBC Radio produced a few audio dramas of Torchwood, that originally aired on BBC Radio 4. The first celebrated the switching on of the Large Hadron Collider. This was followed by one three-episode series of plays to bridge the gap between series 2 and 3, and another, Torchwood the Lost Files to bridge the gap between 3 and 4.
 * BBC Audio made some audiobooks of The Sarah Jane Adventures.
 * There have been a few instances of BBC SitComs transferring from TV to radio:
 * Dad's Army was successfully adapted for radio, running for four series.
 * Steptoe and Son also made the transition from screen to sound.
 * One Foot in The Grave was also adapted for radio, but there were only four radio episodes.
 * Yes, Minister: sixteen episodes, according to The Other Wiki.
 * Given that most of Yes, Minister consisted of verbal repartee, it's surprising that it was on TV before radio.
 * Have Gun Will Travel was one of the few television shows that then had an adaptation for radio, as opposed to the other way around.
 * Various adaptation of Kamen Rider, in tapes.

Multiple

 * Especially in the 1960s and 70s, a huge number of record albums were recorded for children by studios like Power Records. They adapted everything from classic novels like Melville's Moby Dick and Barrie's Peter and Wendy, to superhero stories (some adapted from comics, some original) and TV shows like Josie and The Pussycats, to nursery rhymes and fables from Aesop.

Newspaper Comics

 * A BBC Radio 4 adaptation of the Guardian strip Clare In The Community, currently on its seventh season.
 * In 2010, the Telegraph strip Alex was adapted for Classic FM.

Theatre

 * The Reduced Shakespeare Radio Show, a 6-part series produced by the BBC World Service, is essentially an Adaptation Expansion of The Complete Works of William Shakespeare (Abridged).
 * The Moth, a popular New York true storytelling series, did shows for ten years before releasing the audio as a podcast and then the NPR show The Moth Radio Hour.

Video Games

 * Trails in The Sky has a whole bunch of these, focusing on various characters and expanding on them. Even with the games coming to America at last, we're quite unlikely to see these.
 * Final Fantasy Tactics Advance has a Radio Drama adaptation called Final Fantasy Tactics Advance Radio Edition.
 * Ever 17 has two Drama CDs, After You've Gone and 2035, both being Post Script Seasons.
 * Riviera the Promised Land has three drama CDs--one of a promotional nature (the Prelude disc), one containing mostly gag stories and voiced scenes from the game with a single serious original drama explaining antagonist Malice's backstory (the Epilogue disc), and a third containing a series of long episodic adventures (The Precious Chapter). Even when the first two were in print, it was a nightmare trying to buy them, as none of the Japanese vendors selling them shipped overseas. Luckily for (Japanese-speaking) fans, the third didn't have this problem.
 * Likewise, Yggdra Unison was given a drama CD, featuring three episodic stories starring some of the more popular characters. The cast has expressed hopes of being able to produce more, although we've heard nothing from them on the subject since.
 * The Putt Putt series had a book titled Putt-Putt's Night Before Christmas, which came with an audio cassette of the same name.
 * The Freddi Fish series had a book titled Sing Along with Freddi Fish and her Friends, which came with an audio cassette of the same name.

Webcomics

 * DMFA has an "unofficial" Radio Play here that has currently produced three episodes covering the story arcs "Warrior for Hire," "Recipe for Disasters," and Part I of "The Return of Dark Pegasus," with Part II in the works as of October 2010.
 * Slightly Damned has a Radio Drama adaptation here that currently consists of two episodes covering the first 63 pages, with episode three (set to be pages 64-93) well under way as of October 2010.

Western Animation

 * When Nickelodeon cancelled Invader Zim, seven of the unfinished episodes already had their voice tracks recorded, so they were later released on the (now out of print) Special Features DVD.
 * My Little Pony Friendship Is Magic has two German albums in CD and MP3 formats, each album containing two Freundschaft ist Magie episodes. Volume 1, Ein Auftrag von Prinzessin Celestia, contains "Ein Auftrag von Prinzessin Celestia (Teil 1)" and "Ein Auftrag von Prinzessin Celestia (Teil 2)". Volume 2, Apfelschüttelernte, contains "Eine Freundin hat's nicht leicht" and "Apfelschüttelernte".
 * The Wallace and Gromit book Anoraknophobia was adapted for cassette featuring Peter Sallis as Wallace.