The GBA has GBC hardware to take care of running GBC games.
What happens is that some (well, a lot) of GBA games use the GBC sound chip (included in the GBA) to generate part of their sound. So a GBA emulator needs to emulate the GBC sound hardware as well--why you see people talking about "GBC sound". It's not sound in GBC games, but rather sounds fron GBA games that use the GBC sound chip
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