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.

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.}