I also started having this issue about a week ago.  The only semi-solution that I have found is that the audio will improve if I switch the input for the mic from GID6B to Internal.  

Sorry for additional comment - this forum post accurately describes the issue and workaround back in November.  

https://productforums.google.com/forum/#!topic/pixelbook/yNoCwUV8GHI

This sounds like the same issue I posted back in March here: 
https://bugs.chromium.org/p/chromium/issues/detail?id=799623#c7

On the Pixelbook, when a BT headset is connected and is selected for both input and output, ChromeOS will randomly DOUBLE the mic gain of the BT headset mic when it connects to the device.
When that happens the captured sound will be distorted and in terrible quality to the receiving party.
This happens randomly when the BT headset is being connected to the device. Sometimes simply disconnecting the headset and reconnecting it fixed it and lowered the mic gain by half.

I tested and verified this by downloading a sound meter app from the play store.
When audio is being captured correctly, the app would show 30db in a quiet room.
When ChromeOS randomly doubles the gain for some unknown reason, the app would show 55+DB in a quiet room when absolutely no noise. This is obviously a serious audio bug that is not being addressed.

Sadly, although I reported it in March, no one from the ChromeOS team seems to think there is a problem. This is the fourth post I have seen about this, yet this is being ignored.

This is not just Pixelbook issue. I happened on the Asus C101, the Samsung Chromebook Pro and probably other devices.

I'm hoping people who are CC'ed on this issue can take a look at this.
If logs are needed, please send instructions how to capture the logs when this happens and I'll gladly do this.

Power wash / factory reset seems to have fixed this issue.