The machinegun on the Flying Gunship is incredibly loud. Drastically louder than the regular machinegun and most other sound effects in the rest of the game. The rocket launcher is fine and I haven't tried the others, but machinegun genuinely startled me earlier today.

The Flying Gunship in general seems pretty loud, as its engine sound is far louder than that of the Flying Cargo Ship. I'd suggest a Mod Options setting for sounds, but I don't know how feasible this would be. If it affects other weapons (like the Rocket Launcher), it might be hard to balance with the sound file volume difference.

edit
Having said that... After experimenting with volume settings for a bit (hand-editing the Lua files), I found that volume = 0.2 for the gun sounds (down from 0.8) puts them closer to the rest of the game's sound effects, while volumeSettings0.075 (down from 0.1) puts the Flying Gunship engine closer to that of the Flying Cargo Ship. That probably differs from your preference for the sounds, but it may be possible to weigh the various weapon (and engine) sound effects closer together, such that the sound volume setting is more effective.