Posting this here as this is the top search result currently for this issue.
Just Chatted with Google Support for an hour or so - they say they were unable to recreate the issue on their end, and the best they could do is consider the issue in the next update, and explore my warranty options for a new phone.
I don't believe this is a hardware issue, and turning of Switch Access definitely made the volume buttons start working again. However, there's this issue that comes with it -
http://forums.androidcentral.com/nexus-6p/662953-why-do-i-hear-no-sound-when-i-call-ok-google-7.html
In order to get that satisfactory oh-so-necessary "beep" when I say the "OK Google" voice command, Switch Access needs to be enabled (and no on on that thread is entirely sure why that's the case). If I turn Switch Access on, the voice command prompt comes back, and the volume rockers work for a while, but it seems like when I bring up a video in a browser, they'll fail and remain non-functioning. I then have to turn Switch Access off to re-enable the volume buttons, but then I lose the OK Google sound.
Really hoping this gets sorted out soon.