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