Think I might have had a success with this... still testing.
I'm using a white 16gb 4S, got it shipped direct from Apple UK on release. I've had the no outgoing audio issue since I recieved the phone (about 70% of calls IN and OUT) and have been holding out in the hope its a software issue.. audio for Siri and voice recording has also suffered the same problem, not registering any input unless you make a lot of noise close to the mic. Then randomly would come back for the next call or Siri interaction. Tried bluetooth on/off, reset settings, setup as new iphone not back up etc.
Today I tried downloading the iOS5 ipsw from appldnld directly, and forced an option restore to the ipsw in itunes. It completed ok and I set it up as a new phone.
Been using it for about 4 hours now, made 50+ test calls and been using Siri with no problems so far.
I'm really hesitant to call it a fix just yet... keep expecting it to drop out. Would make sense if a batch had got a bad flash in the factory.... I suppose... If it happens again I think I'll be off to the genius bar.