Or just copy your MP3 files in the root of the flash disk,
go to the “if” menu
and add 1-1 condition, that will play that MP3 when you get an SMS or get a call.
Consider: it won’t stop playing, don’t add a long MP3, or you will hear it all day! I’d recommend max 2-3 s length and strongly against cutting your favorite 40-50s from your dearest song.