Recently I upgraded to Ubuntu 20.04, after which I experienced the problem of “dummy output” audio. After leaving the computer to sleep, Ubuntu will set the sound to Dummy Output. A reboot or log out and then login in again always solve the problem. Some quick Internet searches suggest this is a common problem and may be related to the new kernel ( 5.4.0) in Ubuntu 20.04. I have tried solutions that I found on-line but they didn’t solve the problem, including the command “pulseaudio -k,” which was reportedly solved the problem in some cases.
However, running the command “pulseaudio –start” after “pulseaudio -k” solved my problem.