Had a look at the Audio Volume control, but the problem is that it will turn down or up the Volume for all WAV files, and not individually. I have not had time to do this yet, but if you download a free program called 'WavePad' (just make a Google search), you can adjust each WAV file as you wish. You will find them in the main GSX/Sound Folder. WavePad works by increasing or decreasing sound on a % (percentage basis), so if as an example you think one particular sound is too loud, try reducing it to say 80%, pop back in Sound Folder where it came from, and go back into FSX and have a listen.
A word of caution. Please back up your sound Folders before doing any of the above. Easy then to put back if you mess up!!