Also, ISTR the MPEG card only decodes video, relying on the CPU to handle audio. I'd check out lowendmac.com's article/s on it, and on the DOS card. Think there's also an article on the MPEG system at S7 Forever.

*If* it can be done, using the 486 as an mp3 copro would be kinda awesome. Maybe try a DOS mode player- there's a few out there, old and new.