I found a significant issue with the sound-engine:
I began to suspect a serious problem with the FrontEngine sounds in DCS:WORLD, they didn't seem to be working, no matter how you adjusted the PlaneFrontEngine.sdef file settings.
So I went back to all vanilla-standard .sdef config files and to the standard .wav and .ogg files.
But I removed all wind and turbulence sounds and Afterburner, AroundEngine and Back Engine sounds, for all aircraft.
That left just the FrontEngine sound in place on all aircraft, with which to generate engine sound.
I built a mission with lots of different aircraft in it and tested it for sound.
Sure enough, no FrontEngine sounds are emitted by most of the aircraft!
Some do work, however, and these were;
All civil jet aircraft seem to be OK.
All propeller driven transports and Tu95 etc., seemed to work OK.
All jet transports, tankers, AEW, and B-52 FrontEngine seem to work OK.
All other jet bomber FrontEngines do not work.
All jet fighter FrontEngines do not work.
All jet strike/attack aircraft FrontEngines do not work.
There are no engines sounds coming from these aircraft, whatever.
The Su-33 is also emitting a strange noise in this state, sort of like a warbling intermittent APU type sound, especially when in the circuit to land. I checked it a second time and it still did it. It seems to be is the airbrake is engaging and disengaging, several times per second (but not long enough to visably open it ), continuously. I found the F-5 did it sometimes too, but not as much as the Su-33 does.
The Su-27 and Su-30 didn't seem to do it.