Ok, this is not set in stone, this is just the way I have done it in the past.
If you have W.Xp... right click on a sound sample and in the pop up you will see a setting
"RECORD" you can alter the sound volumes by 25 % stages, very eazy way to get the volumes to your own preferances.
The more complex way is to go into the PRESET FILE for each aircraft and you will see the example below for each engine rpm...
[sample.xSakae_2700.wav.rpm]
minlo 1900
minhi 2400
maxlo 6000
maxhi 6500
value 5.0
pmin -1.0
pmax 2700
The value setting seems to have somthing to do with volume,Min of 0.0 and a Max of 6.0 ,BUT
make shure you make a copy before you start messin around in there cos its eazy to **** things up.
Hope all this is of some help to you mate