Black's hdmi connected to an hdmi-2-av external powered device connected to a power amplifier connected to speakers.
In other words: Black -> hdmi-2-av -> amplifier -> speakers
Audio is tested with 'speaker-test'. Speaker-test is started soon after reboot. "Right channel.....left channel.....right channel....left channel...."
The test tone is heard as expected until exactly 10 minutes after reboot.
The only external sign of a problem is no more sound. There are no log messages in dmesg or /var/log/*. speaker-test continues to run, sending text to stdout.
How can I debug?
The faulty black runs this linux:
root@beaglebone:~# uname -a
Linux beaglebone 4.4.30-ti-r64 #1 SMP Fri Nov 4 21:23:33 UTC 2016 armv7l GNU/Linux
root@beaglebone:~# cat /etc/debian_version
I have a second black and it does not have audio problem. It was purchased several months earlier than the above. It is running factory provided angstrom. I don't have it handy to obtain the 'uname -a' output.