At the time, a Google employee on the Pixel support forums stated that it might be caused by "a hairline crack in the solder connection on the audio codec," internal damage that might even be caused by simply dropping the phone a short distance. The search giant supposedly prevented in later phones with tweaks in the manufacturing process, but didn't offer a fix for current phones -- just replacement under warranty.
The lawsuit (PDF) claims that even some phones replaced under warranty experienced the microphone issues -- and regardless of the fixes in place, continued to knowingly sell Pixel and Pixel XL devices with the defect.
"Despite receiving hundreds of complaints shortly after launch -- and admitting the phones have a "faulty microphone" -- Google continues to sell the Pixel phones without telling purchasers about the microphone defect. Moreover, instead of fixing the defective Pixel phones, providing refunds, or replacing the devices with non-defective phones, Google has replaced defective phones with other defective phones, resulting in many consumers repeatedly experiencing the microphone defect," the lawsuit stated.
When reached via email, Google said it doesn't comment on on-going litigation, but it did direct us to a help center page for customers who encounter issues with out-of-warranty devices.