I have reported this issue under Launchpad https://bugs.launchpad.net/ubuntu-mate/+bug/1571322 but have been advised to refer it this forum.
There have been one or two issues with the sound on the RPi 2 & 3 that have largely been solved by the release of a firmware and kernel update to 4.4.9-v7+. Sound from a YouTube video via Firefox appears to play smoothly.
However, when using the Test Buttons from Sound Preferences > Hardware > Test Speakers, the expected response: Front Left and Front Right is invariably incomplete or is not reproduced at all.
This bug goes back to Ubuntu MATE 15.10 but now that 16.04 is released, it is probably a good time to raise this topic again.