I've been using MusicBee for years and I've been using Windows 10 since it came out. This problem started some time in the last few weeks, although I don't know if the issue is related to MusicBee or Windows itself. I don't think it's the driver for my sound card as that hasn't been updated in quite some time.
Previously, if I started, say, a podcast playing in MusicBee and I then left the computer alone, after 10 minutes my monitor would turn off through inactivity and when the podcast finished, at some point my PC would go to sleep.
Now my PC will not sleep if MusicBee is running, nor will my monitor turn off, even if MusicBee is just idle and no music is being played.