I've got the same kind of sound system here, and I've noticed that too. I've been experimenting with different types of ships on the sound....it seems as though some of the ships are 'noisier' than others...like a cruise ship doesn't sound like a wagon full of tin cans going down a bumpy road..it's much quieter...so if I vary my distance from the ship (if I'm in #1 mode), then I notice that the sound level will come up some....when I'm in the bridge, it's the same kind of thing only more subtle.....tugs, motor yachts, and trawlers will always generate more sound than a liner or a supertanker. Even the Agile Solution is pretty loud on the bridge, but if you're outside of it and back away from it...it's very hard to hear.
About the only thing that I'd like to see boosted is the horn/whistle volume and the ambient sea noises.
Jim