Hi, I realise this is probably way to late but I have just worked out how to open these phones as one of mine is playing up. You remove the two screws then inset your fingernail between the two plastic halves and run it arround the phone. This will unclip the small plastc clips that hole it together. watch out as the charging terminas will come out when you do this. I would guess that the solder has cracked on the microphone.
Regards, Andrew
Andrew, December 2007