Ubuntu – 16.04 switches from default audio device when not available, but doesn’t switch back when the default becomes available

There's a bug report about this. In the comments #5 and #36 a workaround is suggested involving prevention of loading a particular PulseAudio module “module-switch-on-port-available” which handles output devices switching on/off or connected/disconnected. Anyway edit /etc/pulse/default.pa and comment out (i. e. insert a #) the line starting thus:

#load-module module-switch-on-port-available

You then need to restart PulseAudio:

pulseaudio --kill; pulseaudio --start

Ah, I figured it out!

tl;dr: the problem was the S/PDIF link between my TV and speakers, which can only carry two uncompressed audio channels.

What triggered my understanding and a very deep dive down the rabbit hole of various incompatible drivers, audio containers/codecs, and cable types was this forum post: http://www.tomsguide.com/forum/59518-6-working-connected-hdmi-optical#561415

Basically, the problem is that when I do that speaker test, Ubuntu is outputting uncompressed 5.1 audio over HDMI. My TV receives that and tries to transmit it to my speakers over S/PDIF, but S/PDIF only supports 2 channels of uncompressed audio! So, it gets downsampled to only 2 channels.

I was fooled into thinking this should work because I've obviously played 5.1 audio over S/PDIF prior to this, but what I didn't realize is that it was encoded via Dolby or DTS to work over S/PDIF.

It seems like my possible solutions are:

1. Continue to output 5.1 uncompressed audio and replace my S/PDIF cable with an HDMI cable.
2. Change PulseAudio to output 2 channel audio and enable DTS/AC3 passthru both in PulseAudio and Kodi, as described here: Enabling 7.1 audio passthru in 14.04 for Kodi

I'm not sure that either has a particular advantage. Option 2 involves fewer cables, but if I try to play a file with 5.1 channel PCM encoding (I imagine that's pretty rare) I'll be out of luck.

Another thing that wasn't obvious to me in setting up PAVU Control is that Dolby technically AC3, and Dolby Plus is EAC3.