Spontaneous Reverb

Often when a character is musically gifted, when they actually show off their abilities, they will gain a reverb.

Reverb is an effect that is often used in recordings to simulate a performing space outside the recording booth. When a sound is made, it will travel through space and bounce off nearby objects. The bouncing sound waves will create a delay effect and cause sounds to taper after they've been released. If you would like to hear what it sounds like, go into your bathroom and make any loud short sound, like clapping your hands. The sound that happens after the clap is the reverb, although in actual recordings, it's rarely that obvious. This is probably why many people believe they sound better when they sing in the shower.

The amount of reverb a sound has depends on the acoustics of the room, so naturally it can't spontaneously change or just happen to be there if it wasn't there before. That won't stop it from happening whenever someone decides to perform.

This often happens because musical performances are recorded separately and in professional studios, so often they will add the effect in because that's what they do. Also we as a population are so used to hearing reverb in all musical performances that we believe it needs to be there for the music to sound good.

Anime and Manga

 * Very common in Anime, especially if the character is young and female.
 * Lala from D.Gray-man
 * Tomoyo from Cardcaptor Sakura, when she sings on-stage (justified because of the acoustics in a performance theater) and during the episode about the Song Card (not justified at all).
 * Azmaria from Chrno Crusade

Film

 * In the film of Return of the King, Pippin picks one up when singing to Denethor (especially noticeable on the high notes).

Literature
""...with that name his voice seemed to grow stronger: it had a full and lively sound, and the dark chamber echoed as if to drum and trumpet.""
 * In The Fellowship of the Ring, when Frodo is trapped in a burial mound on the Barrow Downs, he sings a rhyme to summon Tom Bombadil:

Live-Action TV

 * Shows up in the episode of Xena: Warrior Princess in which Xena has met the slave girl who teaches her how to fight. At one point the slave girl is on the bow of the boat, singing. She brought her own reverb.
 * Parodied in one episode of Cory in The House, where a spontaneous reverb happens whenever someone yells.