I had this issue and just solved it with a complete wipe/factory reset. It's working fine now.
The mic was working fine in video and other recordings, I tested it. It was only outbound calls that were a problem, I could hear the other party fine but my voice was faint and garbled to them. Going into "Safe Mode" improved it, but only partially.
I called Google support and they walked me through the factory reset. It's triggered like this: (after making sure you have your data backed up) You go to shut the phone down by holding down the power button, then when the dialogue box comes up, you touch it and it gives you the option to wipe the device. The rest is automatic. The phone at one point stopped "deleting"and showed a Dead Android Icon on his back... (which was a little alarming) but it then resumed. Once I re-entered all my Google info, the calling sound is now working fine. I think you need a wifi connection to get the phone up again.
PITA, but there it is. There may be a simpler solution. Took a couple hours all told, and I still have some set-up to do. On the bright side, there was an option to choose which of my apps to re-install, and I de-selected some that I had not bothered to delete before upgrading to Lollipop. Upgrading prematurely, it would seem.
I mean, I get it, glitches happen. But seriously Google. The device is first off a PHONE. It has to be able to make CALLS.