I imagine this topic will get moved to standard scripting as it isn't an advanced subject however, as you learned from previous reply, what seems like a good idea at the time rarely applies to ofp. It just isn't another 'me too' shoot-em-up as it's longevity should attest to. So,,,,
music was to be started as you said, when the enemy has actually begun firing on the player,
You will sir, get howls of scream and pain, and, most likely, many, many players will *instantly* type -endmission.
Why? It's termed, disparagingly, 'battle music'. As a player you have a choice of bopping along to some loony tune, or, the choice of listening out for *real* sounds like tanks approaching, or the direction of fire from a semi-silenced weapon, or determining where concentrated fire is coming from or, howls of pain, a chopper flyover, and react accordingly. The moment you drown out the 'realism' of Flashpoint and turn it into just another first person shooter with limited shelf life, it's goodnight, and thank you for playing.
But, by all means, try out your ideas and put in into the beta mission forum for some road testing.