There's a dedicated discussion going on over at Google's Product Forums, and officially, the company is currently in the process of collecting bug reports.
However, in a recent post, a user shared that they were able to resolve the problem by blowing into the bottom speaker. Here's their complete post:
Another user even shared a video confirming the process actually works. You can access the video here. Multiple other users also confirmed the solution works.
So there you go. You can try this workaround to get the mic working until Google officially comes up with a permanent solution to the problem.