Why does the sound automatically mute when an LED is turned on?

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1

The volume mute button on my laptop has a light that normally illuminates when muted:

ThinkPad T14 Gen 2 AMD speaker mute key Enable / disable speakers

I’ve learned that the light’s behavior can be controlled:

led='/sys/class/sound/ctl-led/speaker/mode'

sudo tee $led <<< 'off'          # never on
sudo tee $led <<< 'on'           # always on
sudo tee $led <<< 'follow-mute'  # on when muted
sudo tee $led <<< 'follow-route' # on when NOT muted

The default is follow-mute, but I prefer the inverted behavior follow-route.

Confusingly, something seems to be watching the light and automatically muting the speaker when the light is illuminated. To demonstrate:

play -n synth brownnoise & # The speakers start making noise.
sudo tee $led <<< 'on'     # The speakers go silent.
sudo tee $led <<< 'off'    # The speakers resume making noise.
kill %1

This is backwards; the light should not affect the speaker. It has the insidious effect that follow-route just causes the system to mute itself in a feedback loop.

Why is this happening? I’m not sure what to investigate next.

2

Bit of a shot in the dark but maybe there is a hardware (or possibly Kernel-driver) thing going on that ensures that the LED always shows the mute-state correctly. Similar to some webcam LEDs that always show the state of the webcam.

Now I wouldn’t know why such a LED would be separately controllable or why instead of turning the LED back on it would actually modify the mute-state, but maybe it’s something along these lines.

On my machine (Thinkpad X1), what you do works without an issue.