A new clue: I don’t think it is the entire sound on a track disappearing. Rather, it is the center (as opposed to the sides). I think the reason the lead vocal track seemed to disappear before (but somehow weirdly had some of the effected signal coming through) was that it is a center-panned track.
Here’s the new finding: Tonight I added a reference track to my project – a stereo mix/master, sent only to a Cue send. I played this track just before taking my dinner break. Coming back a few hours later, the other tracks in the project seemed to be working, but, when I played the reference track again, switching to the Cues to listen, it was sounding strange. At first, the acoustic guitars in the intro (probably panned a fair bit to the sides as there were two guitars) sounded semi-normal, maybe not as loud as before. But, once the lead vocal started, it was sounding garbled, like it was under water or going through a fan or something – probably it had some effects, such as chorus on it lightly – and the vocal itself was very faint. But the guitars still sounded about the same as they had in the intro.
I tried switching the track to go to the stereo out and listened through that, and it has the same issues.
If I play the audio file from the Pool via the Sample Editor, the effect is the same. However, if I go out to Windows and just double click on the file to have it come up in the Windows 10 player, it sounds like it should. (I’m guessing it will also be back to normal in Cubase after tonight’s reboot.)
What was special about that track? Probably that it was the most recent track added to the project, though, at this point, the other tracks in the project are instrument tracks, so I can’t say definitively on that front.
I tried dragging some other audio files into the project, and they also get the same issue (i.e. center missing) at this point, so it does seem to affect more than just the one track/clip here. I’m not sure why this is different than what I saw with the other project where only one track seemed to be affected. But, in that case, I also wasn’t adding those other audio tracks after the problem occurred – they’d been there before the problem started, too.
One more hint: If I put the (Waves, I think) S1 MS Matrix plugin in the insert for the track that has this problem, it plays back as I’d expect! So definitely something is getting messed up in the signal related to Mid/Side stuff. I’ve edited the title of this thread to reflect that it is just losing the Mid portion of the sound.