In fact the default volume widget can control bluetooth speakers: on that, see the last part of the answer.
Short-keys can be assigned to the following commands:
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%-
For a panel/tray button, install
In Debian, Ubuntu:
sudo apt install qasmixer.
To have it in system tray, go to File-Settings-System tray and, in case the tray icon doesn't appear, un-check then check again the "Show tray icon" option. Be sure to check also Mini mixer device - Current so that the tray icon actions like mouse scroll act upon the desired device.
Normally it will control volume of all kind of speakers. In case it stops controlling the bluetooth speaker(s), check File-Settings-Startup and View-Show device selection-Mixer device.
Make it start with the system by adding it to startup application list with an argument for
start minimized in tray:
Sources here and here.
And in fact the panel volume widget can control the bluetooth speaker volume, in two ways:
- Right click the panel widget and select Configure...
There, you can switch between the internal or other speakers controlled by the widget. In the same window you can set the external mixer that will be used below. Normally that is
pavucontrol-qt, but there is also
- Left click the panel widget and select "Mixer"
The mixer allows to change the bluetooth speaker volume.