Hi
I do what 'racartron' does (or something similar)
In my Aircraft folder (FS9 - I don't have FSX yet) I have a folder named Sounds
In this folder I have a large number of folders containing engine sounds - each sound folder has a specific name - it could be just 'radial' or 'F-15', or something more detailed like 'A340-200300 CFM56-5CX'
My sounds folder currently has 150 different sounds (about 3.5Gb) but these are shared by a lot of aircraft so I actually save space.
When you have an aircraft that does not have it's own sound, the sound config. file (in the aircraft sound folder) will probably read something like "alias=b737_400\sound" - a lot of airliner addons have this.
If the aircraft was a A340 (with CFM56 engines) I would then change the config to read
"alias=Sounds\A340-200300 CFM56-5CX "
This directs FS9 to my Aircraft\sounds\A340-200300 CFM56-5CX folder and the aircraft will use the sounds in that folder.
You can use the same system for panels too.
Not having FSX, I do't know how it's file system works but it could be similar (or not:isadizzy: )
I hope that this helps.
Alan