The Nintendo Game Boy Advance had a Z80-like processor which was typically used for backwards compatibility with GB and GBC games. According to its Wikipedia page, an uncited comment claims that the Z80 was also used by GBA games to supplement audio (emphasis mine):
Backward compatibility for Game Boy and Game Boy Color games is provided by a custom 4.194/8.388 MHz Z80-based coprocessor (Game Boy Advance software can use the audio tone generators to supplement the primary sound system), while a link port at the top of the unit allows it to be connected to other devices using a Game Link cable or GameCube link cable.
This brings up two questions:
- Is this true? Was the Z80 ever used by GBA games to supplement the audio?
- How did the Nintendo DS (which has no Z80) deal with this when running GBA games?