re-set xfce4-pulseaudio-plugin default after HDMI monitor standby

Question

Context: Debian/testing on a ASRock B450 with Ryzen 2200G, HDMI monitor.

I can set the default audio sink either via /etc/pulse/default.pa or manually with this command: pacmd set-default-sink alsa_output.pci-0000_38_00.1.hdmi-stereo-extra1

However, once the monitor enters standby mode, the default gets reset to the internal HDaudio card and the volume applet is no longer tracking the HDMI volume, rather the internal audio card feeding the headphone and back panel jacks. I assume this happens because the HDMI "card" gets unplugged but I'm not sure.

I also tried the suggestion in this other question but it didn't work. Most questions seems to be about setting pulseaudio defaults but this doesn't work: the audio is still routed correctly to the HDMI monitor, it's just the applet which is tracking the wrong volume. If I reset the default card with the command above, it goes back to showing the proper volume until the next standby.

How do I fix this? Basically, I need to run "pacmd set-default-sink" right after the hdmi sink becomes available again.

New info on sinks

1. before standby: card 0 is hdmi, card 1 is internal; card 0 is default and available, card 1 is also available but not default (link to details: https://paste.scratchbook.ch/view/83c0e9a5)
2. during standby: hdmi becomes unavailable, internal card is marked default (link to details: https://paste.scratchbook.ch/view/9e9ae422)
3. after standby: hdmi is again available, internal card is still default (link to details: https://paste.scratchbook.ch/view/9be5801a)

After interrupting standby, I started a new player and I can confirm the music comes from the hdmi monitor, yet the applet shows a crossed-out speaker.

Answer 1

Ok, what happens is the following:

Each port of a sink has an "available" state, and putting the HDMI monitor into standby puts the port state into "not available".

The default sink can also be thought as fallback sink: It's the sink that gets used of Pulseaudio can't assign a valid sink to an application using the stream cache (where it remembers which stream used which sink). So it can't keep the default sink pointing to something that can't be used for this purpose, which is why the default sink switches to whatever other sink is available.

And when the monitor comes back from standby, the default sink stays, and doesn't switch back (there's no "preferred" default sink).

The simplest way to deal with this would be to write a simple Pulseaudio application that subscribes to configuration changes (see here for an example), and whenever your HDMI sink becomes available again, sets it as default.

Another option may be to use pactl subscribe, parse the output, and do the same in a shell script.

Answer 2

Here is an example how I am parsing the output of 'pactl subscribe'.

In this example the trigger for an action was the string "Event change on card #0" when a headphone was plugged or unplugged.

#!/bin/bash

function MySubscription() {
    if [ "$1" != "Event change on card #0" ]; then
        return 0
    fi

    #detect plug state
    pactl list | grep "analog-output-headphones" | grep "not available" > /dev/null
    STATE=$?

    if [ $STATE -eq 0 ]; then
        echo "headphones unplugged"
        #YOUR COMMAND HERE
    else
        echo "headphones plugged"
        #YOUR COMMAND HERE
    fi

    return 0
}

export -f MySubscription
pactl subscribe | xargs -I {} bash -c 'MySubscription "$@"' _ {}