I had problem with my Lumia 800 where I had no audio sound on any calls - i.e.. I couldn't hear anyone. I installed the Diagnostic App by entering ##634# and ran that through...it didn't identify any problems. I then went through each category individually and eventually ended up in 'Audio Loopback' where I set the Loopback to 'Earpiece'. I then gained control over the audio up/down switch on the side and managed to get my audio sound up to 10...it had been on 0 (?) Whereupon....hurrah...hurrah...hurrah...i got the audio back on my incoming calls.
Hope this helps!