You either:-
A) have too many planes installed. The limit is around 100. Remove some or ''mothball'' them (store them in a seperate folder so you can add or take away planes when you need them). As shadow wolf mentions this includes planes with multiple textures (i.e. each different texture counts as a plane)
B) You have a plane which is missing something. 90% of the time this is either nothing in the sound file or the sound is ''aliased'' to a plane you don't have. Check any planes you added recently and make sure they either have their own complete sound pack or they are aliased to a plane that you have in your aircraft file
(If a plane is aliased the sound folder will only have the sound.cfg doc in it. If you open this up in notepad it should read something like=
[fltsim]
alias=A6M2_Zero\sound
The name should be a plane (folder name) that you have in your aircraft folder)
You may want to change this anyway to a more realistic plane. Personally I have most of the excellent sound packs from Lawdog and OBIO and I add them to every plane I have just for simplicity and because I have plenty of disk space. Instead of Aliasing your plane you may want to do this. All you have to do is download the pack you need (
This thread by Rami is an excellent resource for engine sounds), then you copy and paste everything into your sound folder
If you don't solve the problem after trying these two solutions then I'm not completely sure what it may be so I'll let someone else advise you
