PulseAudio changes default device occasionally

Question

I run Devuan GNU/Linux 3 Beowulf. My system uses pulseaudio. I have an on-board audio device, 2 GPUs with their own audio capabilities (e.g. via HDMI) and on-board HDMI which goes to my monitor (that does not support sound). Naturally I want to use my on-board audio, which is also the only one connected to any speakers.

My choice of "Built-in Audio" Profile, in pavucontrol, is: "Analog Surround 5.1 Output + Analog Stereo Input (unplugged)".

Problem is, whenever I leave the computer for a while, then come back, I notice the choice has switched to "Digital Stereo (HDMI 2)" for the output.

My questions:

1. How can I make PulseAudio stick to the same choice of profile, even if additional hardware appears or disappears?
2. Why is this happening?

Note: Devuan 3 is essentially Debian 10 (Buster) without systemd, so solutions for Debian Buster should apply.

Answer 1

This is most likely happening to you when you leave the computer for a while, then come back. Why?

• You have a monitor connected via HDMI.
• HDMI supports carrying audio, and is thus an audio output device.
• When the monitor powers off due to inactivity , then back on, this audio device becomes available again.
• PulseAudio is configured to switch its default output device to any device becoming available.

How to stop this from happening? Edit the file /etc/pulse/default.pa

and comment-out the line:

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

by prepending a hash mark (#) to it.

_{Credit: This AskUbuntu answer.}