[Guest]

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?