yes, you could down mix the mp3 to mono 22khz AIFF.

then use SFPLAY to play it...  

the only deal is your mac has to have a Apple Sound Chip.

So Any Mac Portable or PowerBook will work.

So The Mac II and so on will work

So the LC and later will work.

The SE/30 and the Classic II will work.

For machines like the  128k / 512k / PLUS / 8mhz SE / Classic I

you are stuck with 4 channel midi at the best, and i'm, sure there is a ( close enough ) .midi file out there somewhere that \

probably even sounds ok.... 4 channels was still pretty snazzy even for the 128k of the day.

I'm sure amiga guys though have a good laugh... but its ok.