Iphone low microphone volume, any fix?

You can replace the microphone your self by buying it from a place like this. If you are good at handy work than I would suggest this if your warranty is expired. If it is not out of warranty, I would take it into a AT&T or Apple Store they should be able to fix it as long as they can verify the problem. It may also just be that you aren't talking loud enough or you are pointing the mic away from your mouth. On your old phone, were you getting complaints like this one?

David

Check for dust and lint on the bottom where the mic holes are.