Check this teardown guide. Go to step 17.
https://www.ifixit.com/Teardown/iPhone+4+Teardown/3130
The image I'm going to use is borrowed and edited from the link above.
That chip. Reflow it. Once you're done it should be good. If you're afraid to reflow it then tear it down - stick the board in rice overnight - put it back together. That should get out all the moisture.
If your issue is with the MIC then reflow that chip. If it's not the mic then just do a general reflow with your heat gun and some liquid flux.
*EDIT*
I am sure I edited that picture but for some reason it's not showing. The chip I'm talking about is the second large one from the left. Two large ICs are kind of corralled or enclosed in the metal frame on the left - it's the second chip from the left.
While it may not be related to the MIC - I have had success doing reflow on that chip for those who have issues with the microphone even after replacing the actual mic. Some persons stuff paper back in there to make the chip press down to the board so it makes contact. You can check it out on YouTube or such.
https://www.youtube.com/watch?v=oAAda5Xj3-s
Last edited by khat17; Apr 9, 2015 at 09:47 PM.
Knowing the solution doesn't mean knowing the method. Yet answering correctly and regurgitation are considered "learning" and "knowledge".