Strange - really your .mp3 should play fine, perhaps the SOUND PLAYING check is running really fast in your main loop and causing a delay that is emalgamating the problem.
Perhaps try something like this:
Play music 1
Do
If rnd(100)=1
If music playing(1)=1 then play music 2
endif
sync
Loop
That would only check the music playing about once a second instead of constantly, so it should'nt affect playback very much at all. Perhaps post the code from your main loop, so we can check for anything not obvious that might cause the problem.
Van-B

Put those fiery biscuits away!