We noticed it, and it seems P3D treats sounds for ground vehicles differently than FSX so, the same vehicle sounds louder in P3D. It's not entirely clear if it's an intended behavior or just a bug that might be fixed.
Note that, some sounds are made by the sim itself, so you must control them using the default sound sliders, but others are controlled by GSX, so you must set their volume using the volume control in the GSX Settings.