[Nexus 5 Q&A, Help & Troubleshooting] Globespy:
A possible fix from Google support

I too have become increasingly frustrated with the mic/speakerphone issues - affecting approx 1 in 4 calls.

I don't recall having this issue when I first got the phone when it was on 4.4 (I got mine the 1st week of release).

Anyways.....I called Google support today and explained the issue. They were aware of it and sent me these troubleshooting steps which are supposed to fix the issue. Even if you are using a non-factory ROM you should still be able to follow the instructions to see if it works. If not, might be worth going back to factory image. Thought I would post to see if they help others. I'll report back on whether or not they helped me:

Thank you for contacting Google. My name is Jasmine we spoke over the phone. I understand you are having a problem with the microphone on your Nexus 5. I'm happy to help.

Here are some troubleshooting steps that should be helpful .

If you are having any audio, speaker, or microphone issues with your device, please follow the steps below.

Step 1: If your device is rooted or modified, revert your device to original factory image.

- Troubleshooting steps may not work on rooted/modified devices.

- You can re-image your device with the instructions from the following link: https://developers.google.com/android/nexus/images

Step 2: Check for system updates.

- On your device, go to 'Settings' > Scroll down to 'System' > Touch 'About phone' (or tablet) > Touch 'System updates'.

Step 3: Remove obstructions.

- Remove any cling film, stickers, covers/cases/bumpers, or anything else that can be obstructing the device.

Step 4: Restart the device in safe mode.

- With the device on, hold the 'Power' button to get the 'Power off' prompt.

- Touch and hold 'Power off' until the 'Reboot to safe mode' prompt appears.

- Touch 'OK' to reboot in safe mode.

- Note: To exit safe mode later, simply restart the device normally.

Step 5: While in safe mode, turn off Bluetooth.

- Swipe down from the stop of the screen with two fingers to pull down the quick settings menu.

- Touch 'Bluetooth'.

- In Bluetooth settings, if Bluetooth is on, touch 'On' to switch Bluetooth off.

Step 6: While in safe mode, make sure the volume is on.

- Swipe down from the top of the screen with two fingers to pull down the quick settings menu.

- Touch 'Settings'.

- Under 'Device', touch 'Sound'.

- Touch 'Volumes'.

- In 'Volumes', slide all three indicators all the way to the right to make sure all sound is on and at max volume.

- When each slider is adjusted, a sound will be heard.

Step 7: Check if the issue is resolved.

- Play with the volume levels to hear sound.

- Make a test phone call.

- Use Google Now to test your microphone.

If the above 7 steps didn't work, a factory data reset may resolve the issue:

1. Go to 'Settings > Backup & reset'.

2. Touch 'Factory Data Reset'.

3. Touch 'Reset'.

After the factory data reset is complete and your device reboots, sign into your device using the same primary Google Play account you used to sync your purchased apps and other content.

If your issue has been resolved, great! Let us know.

If performing a factory data reset did not resolve your issue, let us know.

If you have any other questions or concerns feel free to respond to this email directly or contact us at 855-836-3987 or you can visit our Google support website at http://support.google.com

Best regards,

Jasmine

The Google Support Team