Recently, we've occasionally been encountering an issue that seems to involve (or be caused by) the Netflix app on our X1 box.
Sometimes it'll happen when switching to the next episode, other times (like tonight), it'll happen simply by pausing the show for only a few seconds. Suddenly, there will be no audio, at all. My receiver will say "Stereo" instead of "Dolby Digital" "Dobly PLII", etc. Restarting the receiver doesn't do anything to alleviate this.
Worse yet, if you exit Netflix, no channel will produce any sound at all. The only fix to watch TV again is to restart the box (tonight, this was not possible, as we had a recording in progress - will see tomorrow if the audio works on it, or if it cuts out right around when this all happened).
This only seems to happen when something in the Netflix app messes up, but it's annoying to have to restart the whole box to fix it.
Worth mentioning: the Xfinity Assistant did seem to "notice" an issue with our TV viewing on it's own, without me mentioning that (I declined the system refresh, due to the aforementioned recording). May not be relevant, or may be.