Classic British Flight Sim

I recently purchased FSX Steam version and after running it a couple of times I suddenly appear to have lost the sound. Not all of it just the aircraft. Any ideas anyone?

All aircraft, or just a specific one?
Check the sound settings are pointing to the correct sound device.
You could try deleting the FSX.cfg (back it up first) then let fsx create a new one.

All aircraft Dave. All the other sounds are there like ATC and AI traffic. I'll try what you suggest.