Despite mpv and mplayer are both installed, I always get the pop-up “sounds don’t work without mpv or mplayer”, at each card. I am under Ubuntu 16.04 and Anki 2.1.35
Maybe your mpv is too old if you installed it from the standard Ubuntu repository. Install it from ppa:mc3man/mpv-tests
But since you have mplayer installed, it shouldn’t matter. Maybe open a Terminal window and play the audio file with mplayer. Just to make sure that mplayer is working fine.
Do you use Anki from Ubuntu repositories?
Try tar.bz2 version from Anki main website.
and try this add-on: No Sound Fix 1