Just about anything that plays audio will start to distort the media that it's playing just a bit if you overextend it on the volume scale. Certain audio-playing items are only made to go up to certain volumes, and that is, for the most part, the end of the story in most cases. When it comes to the Google Pixel and Pixel XL, however, new reports are surfacing that indicate there may be some trouble in this regard. Most phones cap off the volume at a level that would avoid wear and tear on the speakers, and prevent any distortion, crackling, popping, or other such issues with audio playback. Users are reporting exactly these sorts of issues with their Pixel and Pixel XL units, and then some.
Reports are coming in from Pixel series owners that the issue persists across output devices; anything from a high-end pair of Sennheisers to a Cast-enabled TV can reproduce the audio issue, if the phone casting the exhibits the issue, meaning that the issue does not lie with the phone's speakers. The issue has also persisted across devices, for some users who have gone to Google over the issue and gotten replacement devices. Reports are also saying that booting up in safe mode has not solved the issue. People tested the same apps that were showing the issue on other devices as well, with the obvious conclusion being the the Pixel and Pixel XL were the only phones doing this.
The custom ROM world seems to be quiet on the issue, with a minimum of topics related to audio in the first place, and seemingly no mention of the audio quality issues plaguing most of the Pixel-owning populace. While this strange phenomenon is no guarantee that Google's official software is the culprit, if not a recent update, things seem to point in that direction. For the time being, Google has taken notice of the issue and is currently investigating it, according to the community manager for the Pixel section of Google's product forums. Once Google figures out what's going on, they will reportedly be in touch with affected users, though what action they will take to correct the problem will likely depend on its nature.