its really all about atmosphere, but that would be a good example. say you are crawling thorough a town at night stealthily, the script would check if someone is close, and a slow heartbeat type rythm could start to simulate tenseness. if another unit comes close, the beat could speed up and a bit more eratic, if a thrid is close, the beat would be very fast and tense. say one leaves the area, the beat would go a bit slower, but still tense and less for every unit that leaves your area, until it is a steady, calm beat. say you are spoted when a guard is around, the beat would be more "combat" style, like metal gear solid until there are no more guards around.
also, if it was a stealth mission, you wont be shooting much, so if you do have a sniper rifle and take a shot, there could be a "cliffhanger" sound for a few seconds if you know what i mean, what could either fade back into calm if you are still undetected, or change to a more manic beat if you are detected.
i can explain this a bit better in my next post anyway, i gtg now, but do you get the idea? could be pretty cool.