The memory requirement is actually not a problem, because you may be able to stream the mp3 from a harddisk ( easily 159 KB per second from a 2.5 inch ide disk when used on a 7mhz 68000 of amiga 600) or maybe even from a floppy ( 10 KB per second on a double density floppy ).

The actual problem is that mp3 decoding requires lots of math, and the total cpu usage to decode at 22Khz mono is the equivalent of a 68030 running at 50mhz, which is more or less 5 times as much CPU as a 68000 running at 16mhz.