Google says the Pixel audio issue is hardware-related according to a screenshot of an email exchange between a customer and the Google Store Support team. The audio issues with the Pixel have been ongoing for the better part of a month, with at least one reported problem surfacing back around December 19th, displaying the audio problem in a video posted up to YouTube. At that time a post was also created on Reddit that garnered quite the set of responses from other users experiencing the same issues and a few days later, it was discovered that the Google Store Support team was offering a refund to a customer who was having problems with audio on their Pixel XL.
Interestingly enough, this new email which shows it's from January 13th states that Google is not currently issuing refunds for this particular hardware-related audio problem, but that they would be more than willing to issue a warranty replacement. This is a complete 180 from the previous email that appeared back in December, which may suggest that the act of a refund was either not something the Google Store Support team was actually prepared to do and the response was incorrect, or that the time for the refund has passed. Considering the latter, it's not shocking that that may actually be the case as the first email denoted that a return/refund process would have to be initiated within 14 days, and January 13th was well past that mark.
Nevertheless, it seems now that there is some clear understanding that the audio issues that Pixel owners were having are related to hardware and not software like was previously thought. The customer mentions that Google Tech Support had stated it was a software-based problem that was affecting all Pixel and Pixel XL devices, but according to Google Store Support this is not the case as they mention that it is only affecting select builds of the device, though they do not state which. If the audio issues are truly hardware-based as mentioned most recently as of a couple of days ago, then the unfortunate thing is that anyone who does have an affected device is not going to get a simple fix through a software update, and instead would have to initiate a warranty claim to receive a replacement device. The other unfortunate detail is that Google has not mentioned which builds are affected and how users can avoid getting one of these devices as a warranty replacement, although it would seem likely that they would keep record of this particular build, and make sure they were sending out a different build that wasn't affected by the problem.