There are no special rules for the sound of guns
All combat is heard at a Perception DC reduction of 10. While they did make a wondrous consumable to silence guns specifically, it does not actually say they're any louder than the sounds of a swordfight or warhammer striking full plate. It's notoriously difficult to prove a negative, but I've reviewed the material as thoroughly as I am able and found no mention of guns in relation to Perception except a number of instances of trying to hide guns.
Would it actually make sense?
Guns are indeed louder than general combat is likely to be; looking at a real world chart of (estimated) noise levels, a sledgehammer* runs in around 120 dB, not accounting for hitting hollow plate... probably closer to a percussion section (130 dB). Modern guns ring in at about 160 dB. From experience, I can tell you that muzzle loaders that most Pathfinder guns "simulate" were not nearly as loud. I would estimate them to be a bit more than half as loud as their modern variants due to the type and packing of the firing powder (about 10 decibels less). Assuming 130 vs 150 dB, guns are still four times louder than their loudest allies' efforts.
*Unfortunately, I could not find any information about the sounds of using medieval weapons
GM Recommendation
When I've played games with a Gunslinger, it has rarely come up how difficult it was to hear them. In the times when it did, we deemed it sufficient to drop the DC by another 2 (5 when it involved a Modern Firearm). It never caused a difference in the actual results of the rolls, as everyone was able to discern the sounds of combat itself. I would only say, sometimes sound levels are hand-waved when the party ambushes an enemy which would not be possible if a Gunslinger fires a volley.
