I've seen this before but it somehow got fixed. It's now come again.
Say I'm using Spotify in the background and I start TeamSpeak. TeamSpeak uses the "Call Volume" of my phone's speaker, and seems to want to retain control even when I go back to Spotify - when I use the buttons on the side of my phone, look what happens on the screen ("Call Volume" is changed rather than "Media":
Even after TeamSpeak gets "closed" Android doesn't seem to want to give me back control over the music volume and use the Media volume - I have to force close the app from the background, which isn't ideal and adds steps. I treat this as a TeamSpeak bug, because all other apps work as they should.
I'm on Android 7.0 using the latest TeamSpeak version.