I have a Mac and an AirPort Express on the same ethernet network. I use AirPlay to play music from my Mac on it, and it works great.
Unfortunately, if I pause my music for more than a minute or two, the Mac turns off AirPlay, and switches back to using "Line Out" all by itself.
It's done this with every recent version of macOS (10.11-10.13, at least).
Is this a known problem? Is there a way to get my Mac to stop turning off AirPlay?