Glitch audio or glitch music refers to any form of unintended sounds or music associated with glitches and/or corruption.
In video games it can refer to audio that was not programmed into the game, as well as corrupted music from physical damage of the ROM or arbitrary data interpreted as sound.
Glitch music with an existing basis
Some glitch music may sound similar to existing music due to being derived from an existing soundtrack.
For example, in Pokémon Red, Blue, and Yellow there are many index numbers for music that bring up glitchy music channels isolated from the full soundtrack. These may be accessed from the cries of specific glitch Pokémon, which also vary based on the current sound bank (hence for example, a glitch Pokémon's cry in Pokémon Mansion [bank 1F] may be different to the cry in Cinnabar Island [bank 02]).
On rare occasions in Pokémon Yellow it is possible to hear glitch Pikachu cries which incorporate existing voices but may also have "pure glitch music" elements, such as from encountering the glitch Pokémon 4 4 (hex:BF) in TheZZAZZGlitch's video 4 4's true cry.
Additionally in Pokémon Emerald, a Decamark specific glitch caused by glitch Pokémon 0x0A0D allows the player to effectively disable the "Direct Sound A" and "Direct Sound B" music channels, causing unintended music to player.
Pure glitch audio
"Pure" glitch audio and/or music refers to music that isn't derived from an existing soundtrack. Two examples of pure glitch audio is glitch music caused by Yellow MissingNo. and glitch music caused by accessing a Glitch Hell.
Pure glitch audio may include glitch audio from a glitch sound bank.