Open Notepad and type this:
#loop
playMusic <music name>
goto "loop"
Then save the file as Battlesound.sqs (you must save it as *.sqs, not *.sqs.txt, you can do it in the saving screen in Notepad by selecting "all files") in your mission folder.
After that, in the trigger activation field, you write that:
[] exec "BattleSound.sqs"
That's should work, this is my way.