Unlike the original release, the view position actually tilts forward and backward as you look up and down, since this is more natural looking, and was added for the head mounted display feature. While it works fine on its own, it doesn't take collision into account. Walk all the way up to a wall, put your back to it, and look all the way up. This will cause your viewpoint to go outside of the world, causing HOM effects and will mute all sound as long as your head isn't in the world.

Sorry to bump this, but it somewhat bothers me that I haven't gotten a response from anybody on this. I sent John Carmack an email about this, no response, I sent Brian Harris a tweet on this, no response, and I post it here in the official bug reporting area, and I get no response from the staff. All I want to know is has this been acknowledged by the team working on the upcoming patch, and are they going to be looking into solutions?

It seems like a fairly easy oversight to fix. In my opinion, a good solution would be to add a collision model to the viewpoint, probably a sphere with a radius of maybe 4 or 8 map units. Then the viewpoint would never go outside of the world past that collision radius.

Just repro'd this with standard settings. Disappointing you didn't get an acknowledgement on it indeed; it's a fairly fundamental mistake.