Whenever I run music app in backround while playing music, the music gets quieter.

[Arctherus]

First time I noticed this issue. It seems to have been doing this in recent days, maybe 2 days tops. Any suggestions to fix this?

 

[Mr.NK]

A restart had always been fixing this problem on my Win8-PC, but it never occurred on my surface. You also could try to deactivate the exclusive control of apps over the sound chip (right click on speaker symbol on desktop / playback device / properties / extended (last tab))