Google acknowledges the audio issues affecting the Pixel and Pixel XL
A bug report regarding the problem was submitted to Google's Pixel User Community on Oct. 24, but it is only now that the problem became more widely documented, that Google has acknowledged it officially.
The Google rep in charge of the Pixel User Community has replied to the original post, saying that the issue is being investigated:
It is currently unknown whether the audio distortion is caused by a hardware issue in the Pixel and Pixel XL or if its rooted in software. If you are currently experiencing similar problems on your Pixel device, you are likely eligible for a replacement. However, judging by the various user reports on Reddit and Google's official Pixel Community page, even replacement units may exhibit similar problems so your best course of action for the moment may be to wait until Google issues an official statement on the matter. We will keep you updated.
sources: Reddit (1), (2), Google