Sound is no longer played from the monitor connected from the desktop PC with the HDMI cable. When I checked the output device of Settings-> Sound-> Output, I was in a state where I could only select "Digital Output (S/PDIF) -Internal Audio" (devices connected via HDMI do not appear in the options).
OS: Ubuntu 20.04.01 LTS
Check device
$ sudo ubuntu-drivers devices
== /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0 ==
modalias : pci:v000010DEd00001E04sv00001462sd00003712bc03sc00i00
vendor : NVIDIA Corporation
model : TU102 [GeForce RTX 2080 Ti]
driver : nvidia-driver-440-server - distro non-free
driver : nvidia-driver-450-server - distro non-free
driver : nvidia-driver-450 - distro non-free
driver : nvidia-driver-418-server - distro non-free
driver : nvidia-driver-455 - third-party free recommended
driver : xserver-xorg-video-nouveau - distro free builtin
Checking the sound card
$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC892 Analog [ALC892 Analog]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 1: ALC892 Digital [ALC892 Digital]
Subdevices: 0/1
Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 3: HDMI 0 [HDMI 0]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 7: HDMI 1 [HDMI 1]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 8: HDMI 2 [HDMI 2]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 9: HDMI 3 [HDMI 3]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 10: HDMI 4 [HDMI 4]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 11: HDMI 5 [HDMI 5]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 12: HDMI 6 [HDMI 6]
Subdevices: 1/1
Subdevice #0: subdevice #0
From the conclusion, it was solved with one command.
$ pulseaudio -k
(It seems to be a command to kill and restart the audio system)
Reference: https://askubuntu.com/questions/1256043/no-sound-on-hdmi-ubuntu-20-04
After executing the command, when I checked the settings, an HDMI device appeared, and when I selected it, the sound was played.
I saw the following article and tried to reinstall the driver, but it didn't work in my environment. https://qiita.com/soh506/items/b425aa3001b58edbea5b