[Bimmer]

I've checked the Allied planes so far. The following is a list of aircraft that have problems with the engine sounds. All of them do play the engine sound in the external view, though the volume is very low. Internal sounds played are either the engine starting sound only, or no sound whatsoever.

A-20C, A-20G : Start sound only
B-25C, B-25G, B-25H, B-25J : Start sound only
Buffalo Mk.1, F-2A2, B-239 : No engine sound
TBF-1, TBM-3, Avenger Mk.III : Start sound only
P-51B, P-51C, P-51D5, P-51D20, Mustang Mk.III : Start sound only
SBD-3, SBD-5 : No engine sound

Perhaps if someone with more knowledge of the sound files associated with these specific planes can make sense out of this we can resolve the issue.