Can't Unhear It

The tendency when enjoying a story to "read" or "hear" a character's voice depending on the observer's preference. More frequent when a character does not or did not officially have a voice (such as a book), especially until some much later adaptation.

Movies are frequently cited (accused) as creating the assumed voice of a character even when there's no objective reason this should occur (e.g., it is simply the actor's normal voice or an actor doing his own interpretation). If the work is frequently adapted, this is usually based on the most popular actor portraying the role. Many writers do in fact "hear" and "see" their characters but generally don't feel the need to enforce this view on the audience except in Broad Strokes.

Interestingly, why one voice is locked into our minds may not be related to how good, official or genuine it is. It might be that the "official" or mainstream interpretation is seen as pretty terrible or disingenuous by the observer. Maybe the audience remembers an obscure adaptation which existed as the sole version until recently as a childhood memory, or maybe there was something merely memorable (or infamous) about said "voice".

This is a Subjective Trope, of course...

Not to be mistaken with Brain Bleach. Also, has nothing to do with Mondegreens.

Anime and Manga

 * An often obscured argument in the old "dub vs. sub" wars is that some audience members prefer a character they largely can't understand simply because of voice intonation, while others prefer a newer interpretation if the original is seen as an overdone Pigeonholed Voice Actor.
 * It's even possible, with a sub, to "hear" the character speaking English in the non-English VA's voice when you try to remember it, because you were paying attention to how the voice sounded and the English subs, not the exact sounds the voice actor was making.

Comic Books

 * Regardless of which version you think is best, Mark Hamill's interpretation of The Joker is often the one comics fans will hear in their heads when reading his lines.
 * And Kevin Conroy's Batman.
 * Add to that David Warner (Ra's Al Ghul) and Micheal Ansara (Mr. Freeze). The show was so full of iconic voice acting, it might be easier to list the ones where they didn't become "the voice".
 * Also coming from the DCAU, Clancy Brown's interpretation of Lex Luthor is considered to be the voice of him.

Film

 * Sean Astin's distinctive accent for Samwise Gamgee in the Lord of the Rings films is very similar to one of the most famous (extant) audio narrations of the book, though Astin claims he wasn't aware of the audio version. Eerily, Sam is never actually written with such an accent in the books, making it all the weirder.
 * Also, Ian McKellen as Gandalf. His voice has become Gandalf's voice.
 * Christopher Lee also fits this trope perfectly, the man was born to be the voice of Saruman.
 * The roaring, gravelly George C. Scott's legendary performance as the titular general in Patton completely belies the real-life George S. Patton's weak, thin voice, which served to make the general somewhat unfond of oration.
 * Alan Rickman's performance as Snape in Harry Potter was so memorable even the novels' author J. K. Rowling couldn't unhear it eventually.
 * The same was true of Evanna Lynch's Luna Lovegood -- like Rickman, she displaced and became Rowling's mental image of the character.
 * Regardless of your opinion on the TV adaptation on Terry Pratchett's Hogfather, Michelle Dockery and Marc Warren's performances as, respectively, Susan Sto Helit and Jonathan Teatime are probably going to be definitive. Especially Teatime.
 * Try reading The Maltese Falcon after watching the film without hearing Humphrey Bogart, Peter Lorre, and Mary Astor in their roles.
 * Simply mentioning James Earl Jones is enough to make someone think of Darth Vader's deep and menacing voice.

Literature

 * Some of the voices that Jim Dale gives the characters of the Harry Potter universe can be, for some, quite hard to unhear. Snape and Umbridge being the worst offenders.
 * This is also true of Stephen Fry, as he does the British audiobooks. Try and read the first Mrs. Figg scene in Order of the Phoenix or Ron's love potion scene in ''Half-Blood Prince without remembering his hilarious performances and cracking up.
 * Pretty hard not to hear Leo McKern when reading a Rumpole of the Bailey story—especially later ones. (You may even start to hear other actors' from the show in their respective characters.

Music

 * Just try to read the lyrics to a song you know, and read them in your head as if they were a poem or spoken words and not to the tune of the song. Just try it.
 * This is actually a really useful exercise for actors. The trick is to separate the words on the screen/paper from one's memory of the song.

Newspaper Comics

 * Who can read Garfield and not think of the late, great Lorenzo Music? It probably helps that pretty much every voice given to the character since has at least sounded like an imitation of his performance.
 * Those who watched the Dilbert cartoon probably have trouble reading the actual comic without hearing Daniel Stern as Dilbert, Larry Miller as The Pointy-Haired Boss, Gordon Hunt as Wally, Kathy Griffin as Alice, Tom Kenny as Asok and Ratbert, etc.
 * Similarly, readers of Over the Hedge will forever hear Bruce Willis as RJ, Garry Shandling as Verne, and Steve Carrell as Hammy.

Video Games

 * Super Mario Bros Super Show gave the then-voiceless Mario and Luigi accents based on the backstory assumed by the show's writers. For many older fans, Captain Lou Albano's deep but friendly Mario voice still trumps the official "squeaky" one by Charles Martinet being marketed by Nintendo.
 * Anyone who's seen enough YouTube Poops featuring the infamous Mama Luigi episode will never be able to hear this song again without hearing the voices from the opening scene of the episode.
 * Has anyone who has seen the animated adaptation of Pac-Man been able to play the game and not think of Marty Ingels?
 * In Final Fantasy VII, the character Cait Sith was a robotic cat with no discernible idiosyncrasies in speech or diction, expressed or implied. As of the fully-voiced 'Compilation' entries, however, Cait Sith has a thick, rather grating Scottish accent.
 * It's hard not to picture Gordon Freeman not sounding like his Freeman's Mind incarnation should he ever talk, if only because it's the most popular version of giving him a voice. And that's a fan-made rendition of him.

Web Comics

 * Invoked in this Xkcd comic.

Western Animation

 * After a certain Image Board post, it's become memetic that any picture of Farnsworth with the caption "Good news, everyone!" will cause the viewer to read it in his voice.
 * Same thing with the Old Spice Guy.
 * One imageboard-made Drinking Game has one card titled "Yay.", with a picture of Fluttershy. The text: if you read the card's title with Fluttershy's voice, finish your drink.
 * Oh nooo, y'all just read this in my voice.

Real Life

 * Who hasn't ever read a book or an unvoiced game, only to have your preconceived and often cherished notions of the characters' speech or their delivery of pivotal lines and moments just shattered by an adaptation, sequel or update years down the road? The ones you can't ever unhear are the ones only you ever heard.
 * Stephen King conceded that, after seeing Jack Nicholson as Randle P. McMurphy in One Flew Over the Cuckoo's Nest, there wasn't any other way of seeing the character when you read the novel. King cited it as a bad thing, however, as he claims it hobbles the reader's imagination to an extent.
 * Read any quote attributed to Winston Churchill and it should be easy to hear it in his distinctive deep, gravely voice.
 * J. K. Rowling has admitted that Evanna Lynch did such a great job portraying Luna Lovegood in the Harry Potter movies that her own mental image of Luna eventually morphed into Evanna Lynch's performance.
 * Howard Cosell had one of the most iconic voices in show business, his staccato tone and distinctive accent one that is still easily recognizable.