The title says it all; at some point Ubuntu 18.04 stops playing sound and the only available sound device is “Dummy Output”. Apart from rebooting (which fixes the issue) this command executed from the terminal fixes the issue:
pulseaudio -k && sudo alsa force-reload
Contributed by user Kim, source: https://superuser.com/questions/17312/restart-ubuntu-sound-processes-via-command-line
George's Techblog 