Muzzle sounds as well as explosion sounds can be heard far beyond their config defined values (the GMG for example is defined to be 1000 meters but is easily heard 5000-7000 meters away with almost no change in volume).
Furthermore sounds do not seem to be attenuated by physical objects such as terrain either (you can hear GMG fire at the airport on the other side of Stratis). {F18510}
Run included repro mission. Listen for GMG fire from the airport. Move your starting position around and notice there is little to no change in the volume of GMG fire.
I have tried this with my M109A6 from A2 ported to A3 and I am able to hear the gun from over 20KM away on my large testing map when fired (the speed of sound issue is also present).
This bug is apparent currently both on the dev and release stream.