When selecting Sound Preferences > Hardware > Test Speakers, the
expected response: Front Left and Front Right is invariably incomplete
or is not reproduced at all.
Other applications attempting to use the built-in sound system
demonstrate the same broken or intermittent behaviour.
Note: this is using sound output from a Raspberry Pi 2 or 3 via HDMI
and a monitor with built-in speakers. Since the monitor works properly
with sound over HDMI via a laptop running Ubuntu (64 bit), it is
presumed that the Ubuntu MATE (16.04 Beta 2) implementation on the
armhf for Raspberry Pi 2 & 3 is at fault.
I have set (ie uncommented) hdmi_drive=2 in the Config.txt file (as advised by Martin Wimpress) but this shows no improvement.
(What I am attempting to do is to run Gqrx (a software defined radio) on the Raspberry Pi 3. If this works, the use of a radio frequency dongle (eg FUNCube dongle Pro +) would make the RPi into a high specification radio communications receiver. Gqrx runs beautifully on a MacBook Pro under OS X. It is also capable of running under Ubuntu 15.10 on a Dell XPS13 laptop. Its functionality on a RPi would certainly reduce the total hardware costs and be a significant achievement for the RPi and its UbuntuMATE implementation.)