I completely failed at getting sound to come out of my Ubuntu 18 computer from the motherboard, from an internal sound card or from a USB sound adapter. Zero sound from any of those. What finally worked was to get a device that took an HDMI cable as input and converted it into an audio signal on a regular audio cable. This is a 2 monitor setup, so HDMI-0 is my main display and HDMI-1 is my secondary display. There is no third display, but the third HDMI cable, associated with HDMI-2, is used only for sound, so I don't want it to display at all.
This stopped working after a software update yesterday, which included lots of NVIDIA software. Now HDMI-2 is displayed along with the other two, as if it were a 52" monitor, which pushes the other two displays around so that they are mostly unusable. But when I turn off HDMI-2, while it fixes the other two displays, the sound from HDMI-2 stops working. So I'm back to a silent computer.
This is the xrandr command that worked before the software update:
xrandr --output HDMI-0 --scale 1x1 --mode 2560x1440 \ --pos 0x0 --panning 2560x1440+0+0 \ --output HDMI-1 --scale .6666x.6666 --mode 3840x2160 \ --pos 2560x0 --panning 2560x1440+2560+0
I've tried to add an additional output line for HDMI-2, such as
--output HDMI-2 --scale 0x0
...but nothing I've tried works, or is even accepted as a valid command by xrandr. Is there any way (hopefully via xrandr, but any other way that might help) to hide the display but not silence the sound?