lux 13 When you say all aircraft do you mean the default aircraft 3rd party aircraft or every single airplane.
If indeed every single plane sounds like a porp driven plane you got some problems.
each air craft should have its own sound folder most of them have their own sounds within the sound folder. While other planes will you the sound from some other aircraft.
Check each of your aircrafts sound folder inside each folder you should see a sound.cfg file make sure that they are not being allised to some other aircraft. The default B737_400 should have its own soud files within the sound folder.
Have you moved the soud files to the main sound folder of fs2002 ???
If you have moved theses files or you can no longer find them I would suggest that you reinstall all the default aircraft from disk 1.
The only way every plane can sound the same is somehow the sound folders are not correct

Lux 13
It is not necassary for you to uninstall FS2002.
Make this simple test. Open your fs2002 folder then open your aircraft folder,then open the B747-300 folder then open the sound folder within the b737-400 folder you should see several .WAV files and a sound cfg file.
If there is nothing in this folder than that is your problem

You can reinstall the orignal air craft off of disk 1 without doing a complete reinstall. Just unzip the zip file into a dummy folder then move the files to where they belong
If you are still confued PM me and I will try to walk you through it 8)