I've been meditating for a year and using Muse daily for 18 days so far. It's been extremely useful and helpful, but I've noticed one strange thing: I often hear birds, continually, when my mind has wandered off into a daydream. I don't realize it at the time, because I'm focused on whatever the daydream is about, but after I come out of the daydream, I remember that the birds have been chirping the whole time--and they're still chirping. Sometimes the birds even make me realize that I've wandered into a day dream,
This only seems to happen when (a) I've gotten the birds to start chirping before the daydream--but focusing on my breath, and (b) when it's a "light" daydream. By which I mean that if I'm having complicated or anxious thoughts, I don't hear birds. I hear the storm. I only hear the birds during the daydream when it's a pleasant or neutral one--if it doesn't bring up any strong emotions or strenuous intellectualism. Both this state and focusing on my breath seem to allow the birds to continue.
This hasn't been a real problem for me, in terms of strengthening my practice, because, when I realize I'm hearing birds, I take it as a signal to focus on my breathing, rather than as confirmation that I have been focusing on it. And strengthening my practice is mostly what I care about.
But when using Muse a diagnostic tool, it's a little misleading. Unless I misunderstand what "calm" is supposed to represent.