There is no command for that. You'll have to make a small script that basically says:
#top
playmusic "mymusic"
~300
goto "top"
You would replace "300" with the length of your song (in seconds). If you wanted to only repeat a certain number of times, it would look something like this:
_count = 0
_max = 4
#top
_count = _count + 1
playmusic "mymusic"
~300
? _count < _max : goto "top"
This would make it play 4 times.