I can't remember the details, but PocketBook devices are configured to go to sleep if there are no events within some short time period. You need to disable this if you want something to run in the background for extended periods (like a music player). A way to do this was described in some old music playing thread. Maybe you can find the relevant posts if you do some searching. I'll see if I can find anything, as well.
Edit: here is the thread I was thinking of:
https://www.mobileread.com/forums/sh...d.php?t=126631