After switching audio output to a bluetooth speaker the Volume Control widget in the panel cannot be used. A manual setting is needed to select the new output in the widgets' configuration.
I could add a second instance of Volume control widget to the panel and try to use that only for bluetooth, but that looks a bit odd...
Is there some way to make the LXQt panel Volume Control follow the actual audio output?
Can I use a different volume control tool with that panel?
A solution is to use a different tool and remove completely that widget.
sudo apt install kmix
By default the actions on the tray icon don't affect the bluetooth speaker, until that speaker is selected as master channel (righ-click the icon, "Select Master Channel").
It shows no volume change pop-up on mouse actions on the tray icon (at least not in this LXQt).
sudo apt install qasmixer
Resize the window and drag it close to the volume icon.
sudo apt install pasystray
It doesn't show a slider, nor a pop-up, and mouse actions are limited to scrolling volume up-sown (no mute), but it has other useful options like switching the audio output. More here.
Add qasmixer -t (start minimized in tray) and/or kmix & pasystray to startup list.
Furthermore, if the bluetooth audio device volume cannot be controlled through the laptop keys, the following commands could be associated to the keys:
Toggle mute: `amixer -q -D pulse sset Master toggle`
Volume up by 5%: amixer -D pulse sset Master 5%+
Volume down by 5%: amixer -D pulse sset Master 5%-
Sources: Terminal command to set audio volume? and How do you mute from the command line?.
