BTW, I’m the guy who’s been emailing you bug reports for a couple of years.
Before I answer your questions, I should clarify something: in my post above, I wrote that this update is version 4.2, but it’s actually version 5.0.122.
I’ve been hearing this problem with two phones – a Nexus 5 (the 2013 model, not the Nexus 5X) and a Nexus 6P.
Because they’re Nexus phones, they both receive the latest OS updates as they’re released. I first started hearing the problem when version two of the Muse app was released (I think that was February 2016), so both phones were running Android 6.something at the time. They’re currently running Android 7.1.1.
As long as you’re here, I might as well mention a new audio bug I discovered this evening. This one was introduced in the new app update (version 5.0.122) and I heard it while using the Nexus 6P with feedback version 3.3.
I began a session with the new Desert soundscape and started hearing the birds (in stereo!) almost right away. But after about a minute, all the bird sounds stopped and I didn’t hear any more for the rest of the session (the other sounds continued to play normally). Here’s the session graph:
This was a 60-minute session, but I stopped after 20 minutes because I wanted to see if I could replicate the problem. So I started another session with the Desert soundscape, and the same thing happened: 60 seconds into the session, all the birds stopped.
Next, I did some short session tests (halting each session after about three minutes) with all the other soundscapes, and mixing in two additional tests using the Desert soundscape. The bird sounds kept playing when using the other soundscapes, but the birds stopped after 60 seconds in all four sessions when using the Desert soundscape. I know this isn’t conclusive, but this is what I've got so far.