iphone 4 people can't hear me during calls since upgrading to ios7

I've had my iphone4 for many years and never had a problem. I recently upgraded to ios7 and suddenly no one can hear me in calls anymore! I tried formatting it and doing a fresh install of ios7, restoring the phone, and nothing works. If I could downgrade back to ios6, that would be fine, too, but for now, I have to use speakerphone or a headset in order to answer calls.

Anyone else having this problem? It's only since the upgrade. I tried installing 7.0.4 hoping it would fix it, but it's still not working. I also tried just clearing out the microphone space, but that did nothing and it wasn't particularly dirty anyway.

It's not that the microphone on the bottom doesn't work at all - if I am doing a voice memo and blow into it, or scratch the surface of the microphone grate, it will record the sound, but faintly. So it's more like the bottom microphone is now very, very quiet. The top microphone works fine, which is why speakerphone works.

The speaker works fine when playing music.