You'll have to wing it on this one.
The rules give you a DC 0 +1 per 10' to hear normal converstaionl tone, which is what is required for the verbal componenet fo a spell.
A few suggestions:
First, you should only allow a more or less general direction if the noise is heard. You may want to allow spomething more if they are really close, like within 10' or so (of course, 50% miss chance always applies).
Second, apply a circumstance penalty based on the amount of other noise present (they are trying to notice a particular noise) of, say +2 per other noise, here, perhaps per non-silent combatant.
Basically, they'll probably get a direction fairly easily (a 10' lightning bolt should no nicely), but it's really hard to narrow down to a 5' square - in fact, I'd say they could either be really close or triangulate. Otherwise, no chance of an exact location.