Display a notification when the volume of the ALSA PCM control changes.
xfce4-volumed handles that task just fine, but it only watches the master control. In my audio setup I output the audio to an external amplifier via S/PDIF, and the only control that changes the volume of the S/PDIF output is the PCM control.
It requires libkeybinder3, which can be installed with your distro's package manager.
Follow https://pygobject.readthedocs.io/en/latest/getting_started.html to install it for your distro.
See https://github.com/larsimmisch/pyalsaaudio.
I dislike venvs, so I do
pip install --target=./libs -r requirements.txt