What kind of CPU have you got? There are some good MP3 players, plenty of MOD players and some decent video players... But MP3s require a decent cpu - video even more so. As a start, SongPlayer (the older version - written in assembler for speed) is very good for MP3s, CDs, WAVs etc., though the newer version is a little slower. DeliPlayer & HippoPlayer are apparently good for MODs (don't listen to them much myself though), and Action from OS3.9 (think its shareware name is different) does a reasonable job of video for a classic.
Edit: Just saw your post... Yes, a 68000 simply can't crunch numbers fast enough to decode MP3 at all! ideally you want a 68060 for full quality, though you might get away with an 040. Mods should be ok on a 68000, and you might try a hardware decoder such as the MAS Player to play MP3s, but even that will struggle with higher bitrates on a 68000...