Well my friend , i have a similar issue also ie; my microphone doesnt function while i try to place a call through my speakerphone. I could hear the other party what they are saying but they cannot hear me at all. I tried several ways but still couldn't resolve the issue. I contacted apple directly but they said that my iphone 4 is out of warranty and in order to troubleshoot the problem, they would need to sign me up on a $30 plan. I am still trying myself as its not guranteed, that apple would be able to solve the issue a 100%. Meantime you could try browsing through several websites on google that might help you sort it out yourself.

GOODLUCK!!!

Replace the dock connector mic assembly, this mic only works on calls. the top mic works on apps and speakerphone.