Glitch sound banks are invalid music banks in Pokémon Red, Blue, Yellow. These effectively serve as categories for all of the music in the games.
Normal sound banks include 0x02 (overworld), 0x1F (dungeons and title screen) as well as 0x20 in English Yellow or 0x28 in Japanese Yellow which contains the printer error theme, Pokémon Yellow unused tune, encounter with Jessie and James theme and Pikachu's Beach theme.
Forcing a glitch sound bank to play can cause glitches, with not only glitch audio but also glitch effects such as a glitch trade screen (when attempting to play a song from bank 1 in Red/Blue) or freezes (see Too bad! The trade was canceled! (effect)).
Glitch sound banks can be accessed as a side effect of encountering regular MissingNo. in Yellow, or with the GameShark codes 01xxEFC0, 01xxF0C0 where xx is to be replaced with the bank.