Since I updated tot Ubuntu 19.10 my Dell sound bar gets deselected as the main audio device all the time. Apparently there is a fix for that (Thanks Daniel Gibbs for pointing it out, see the source).
Edit (using the terminal, using Nano editor: CTRL-O to save, CTRL-X to exit, for more commands, check the Nano Basics Guide):
sudo nano /etc/pulse/default.pa
Find and comment out the following line (commenting out, use the ‘#’ sign in front of a line, without the quotes):
Restart Pulseaudio (or log off and log on again)