I used a wee CLI program "MPEGA" I had it set up so that I could click the mp3 file in a DiskMaster lister and have it play :-D
this line in the DMstartup file will do it:
"AddAutoCmd ΓΏ,StdIO "CON:0/12/190/100/Playing Mpeg Audio...";extern mpega -I200004 %s;StdIO CLOSE"
and this one will give you a menu Item that will start an mp3 playing:
"AddMenu FILES, MPEGA, m, StdIO "CON:0/12/190/100/Playing Mpeg Audio...";external mpega -I200004 %s;StdIO CLOSE"
these should be all on one line and of course you have to have AHI installed.
Mpega is on Aminet.