I have experienced the exact same problem with my Lumia 800.
When recieving a call, the earpiece turns off and the microphone dies. Switching to speakerphone resolves this issue and the phone works as expected. However, when the call is ended then the main speaker, earpiece and mic become non functional.
When running the diagnostic package (just enter ##634# through your phone keypad if you don't have the tile for it) everything works fine and the phone goes back to normal. Until I receive or make another call.
As everything physically functions, I this indicates a software issue. However,as I have reset to factory settings and the issue remains I'm a little confused.
Has there been any advance from Nokia as to what this issue is? It seems that there are several people effected and a lot of confusion as to whether it's a software or hardware issue. It would be unfortunate for handsets to be returned when a patch could
resolve the issue.
Thanks
Xy