In other 6502-related news, here is a commented disassembly and detailed analysis of Rob Hubbard's music playing code, as seen in numerous Commodore 64 games of the 1980s (and later ripped off by crackers and demo scenesters). If there was a museum of feats of 8-bit computing, this routine would be sitting in a prominently placed glass case in one of its wings.
Yes.
I suspect it may have to do with many Amiga coders having cut their teeth in the C64 demo scene.
No idea if there's a direct lineage, but there's a very definite commonality between this and the .mod format that was the default on amigas.