VGM format
A logging format that records every write to a sound chip's registers with timing, so playback is a faithful re-run on an emulated chip.
Rather than store audio, a VGM (Video Game Music) file saves exactly which values were written to a sound chip's registers, and when. The player feeds the same writes to an emulated chip and gets the original sound back — independent of whatever game code first produced it.
The format grew out of the Sega emulation community in the late 1990s and supports a long list of chips: SN76489, YM2612, YM2151, AY-3-8910, SID and more. The VGMRips archive and the VGMPlay player keep it alive. Compare NSF, which runs code instead of logging writes.