Or get an Acard SCSI IDE bridge and use any available IDE drive up to 2 Terabytes. That should be plenty of MP3s.

Or find a SCSI card reader and a bunch of memory cards (suitable for your card reader) to stuff your MP3s.

Honestly, though, I don't think playing MP3s is the best use of a Quadra 800. Many Nokia cellphones will play great MP3s, so will an older iPod or iPod shuffle.