I am not sure which of the many tweaks I have done but I now have the loud engine/afterburner sounds restored. That said if SimApp Pro is not active in the background it reduces the sound noticeably.
After much investigation I have found a work around.
if I disable eyetracking and auto IPD the headset no longer goes blank thus crashing DCS 10-60 minutes into a session. Note: eyetracking and and auto IPD work fine right up to the crash so it’s not a calibration failure. Historically I had connected the Pimax Crystal data cable to a USB 2.0 (not USB 3.0) which had worked fine for months but now I use USB 3.0. More testing over the weekend and then an email to Pimax on Monday!
UPDATE: Following an exchange of emails with Pimax I have restored eyetracking. The solution was to remove the DP cable from the headset socket and disconnect the short cable from that socket direct into the headset. Then the long DP cable is insert directly into the DP socket under the headset top cover. Thus eliminating the short cable altogether.
Nuggetz's post in F16 Afterburner Sound - Missing was marked as the answer