PS4 doesn't play music in surround over HDMI ARC. Why?

So, my PS4 is hooked up through a 5.1 system via HDMI. Movies and games are in surround, but music through the Spotify app and even menu music in games such as Battlefield 1 play in stereo. Is this by design? Also, side question: my 5.1 supports Dolby and DTS. Does one sound better than the other?