[Revi 01]

have you installed any new planes recently?
more specifically, have you used a new buttons file recently?
afaik, you might get missing engine sounds if you are using the wrong buttons file, which in turn might use different prs file settings, which would account for losing engine sounds on some AC, in my case it was most of the Hurricanes, Spits and Beaufighters, but others lost sounds for different AC.
Which mod version are you using?

Yes installed new AC...but first noticed some sounds on A6M's missing a long time ago. Now probably 1/2 my AC sounds are missing; P47 P40 F6F.
Havent been too concerned up till now as only focused on flying P51.
Using 409b. Unified installer 3.8.
So maybe its something to do with buttons file?