As far as I know, "OFP coding" doesn't work in the .html briefing...
I'm almost positive I tried throwing a playmusic command in there once and it just didn't do anything... You could always try it out though if you want to make sure..
On another note, some missions have a custom sound for when you click on the "gear" tab, I've never needed to know how this works, but maybe you could have that play music or find out how that works and adapt it to your needs... (One of the missions that had this was the demo missions that came with the BAS rangers)
Asmo