Not all MP3 files loop. Usually its automatic if you use the "sound- play music" trigger, but depending on how the mp3 was ripped/created, it may or may not loop. If it doesnt, just create a trigger with a periodic event that plays the music after so many minutes. (i.e. if its a 1:30 minute long song, have the periodic event trigger fire every 90 seconds.)
-VGsatomi