I build Doomsday Engine from source on Ubuntu 18.04.2
For the most part, everything works. However, when I have a USB headset plugged in (thus multiple "output" options) things go awry. I haven't seen any other posts, so I'm not sure if there's something unique to my environment, or if it's just not common for people to have multiple output devices.
As a shorthand, I'll call the audio output I want to use "speakers" and the one I don't "headset." If I set the system output (Ubuntu) to "speakers" then when I launch doomsday, "speakers" will be the selected "Output" (as expected) - however, the sound will actually come out of the "headset" (not expected). If I close Doomsday, and change the system default to "headset" then launch Doomsday, the "Output" is set to "headset" - but the sound will come out of the "speakers."
There's oddity one - in a nutshell, when I have two output options in Doomsday, the sound comes out of the one that's not selected.
The second oddity is that I can't use the "Audio Backend" configuration menu to make a change to "Output." If I select the "Output" option, it will list my choices. However, once I select a different one, I don't get an "Apply" button. It's as if I didn't change anything. If I change this and something else, I'll get an "Apply" button, but using the "Apply" button does not actually save the change made to "Output." I can go right back into the settings, and the original output device is still listed as the selection.
At current, my only options to get sound to come out of the "speakers" is to either unplug my USB headset before launching doomsday or set the system default to "headset" before launching doomsday.
I tried this on the 2.1.0 release, the 2.1.1 release, and also the latest build (3129).