Prior to your post above and after reading through some other topics on the forum, I decided to reload the C++ runtime library 2008.
During the re-installation a dialogue box did pop-up stating that the runtime library needed to be repaired!
Following your input I checked my sounds environment slider and it was set to 90%, now changed to 100%.
Went for a fly and my lack of touchdown sound(s)is now ok. (My problem was for wheels only landing, wet landing was ok)
Not sure what solved the problem, perhaps a combination of both changes, but very happy now.