Similar to FLS' report, I went to desktop while in flight, came back and plane engine sounds were gone, although I could hear other sounds including external engine sounds from other planes.
Mission sound was no different for engines, as I previously believed. It was a spitI that I was flying so the engine would shut off and restart and also WEP did change engine sound (my bad).
However, I found another bug. Ammo Counters do not work for Spit I. Likewise, after I alt - tab to desktop, not only was the sound and ammo counter wrecked, but all of the dash needles and dials disappeared as well. I was able to get the dials and needles back after going to external view (IIRC).