Reference: bugs.launchpad.net/ubuntu-mate/+bug/1501426
Unlike Xubuntu, Linux Mint, Zorin OS Core, etc., Ubuntu Mate fails the basic Firefox HTML5 audio test for MP3 and M4A files. Fortunately, there is an easy fix: install gstreamer1.0-libav
(1) hpr.dogphilosophy.net/test/
(2) html5test.com/
Single file test:
archive.org - Issac Asimov - Foundation Trilogy 1 of 8 MP3
HTML5-only MP3 media web site test:
http://www.gooplusplus.com/jukebox
Sorry about the non-clickable top 3 links - new user restriction is only 2 links