I only have some little experience with Ubuntu but today I switched to Mint.
With Ubuntu I had to edit /etc/pulse/daemon.conf and set default-channels to 6 ans enable lfe-mixing.
But in mint this doesn't work.
If I manually switch to 5.1 soundchanneld at the sound-settings in the control centre, everything works until I stop als play music again. It's exactly the same behavior I have had with Ubuntu.