Axus Xonar DGX SoundCard

Setup

Enabling Digital Audio

After connecting the PCI card to the computer, and turning it on, confirm that the card is visible to the kernel using

lspci    | grep -i oxygen hd audio
aplay -l | grep -i xonar dgx

alsamixer
   F6              # choose XDG Audio card
   Analog Output   # MultiChannel
   S/PDIF          # press M to unmute

WARNING:

Any audio that is already playing will likely be set to the motherboard sound until reboot, or manually changing the stream's output (see the tool pavucontrol-qt)

Setting Default Card/Sink

If the above was not sufficient, then you'll likely need to manually choose the default card.

Determine the default output (has a * next to name)

pacmd list-cards | grep -e 'name:' -e 'index' -e 'alsa.card_name' -e 'active port'
pacmd list-sinks | grep -e 'name:' -e 'index' -e 'alsa.card_name' -e 'active port'

  * index: 1
   name: <alsa_output.pci-0000_00_1f.3.analog-stereo>
      alsa.card_name = "HDA Intel PCH"
   active port: <analog-output-speaker>
    index: 4
   name: <alsa_output.pci-0000_05_04.0.analog-stereo>
      alsa.card_name = "Xonar DGX"
   active port: <analog-output-headphones>   # optical, headphones jack, line out, ...

Set the default card

pacmd set-default-card 0   # choose the default card
pacmd set-default-sink 0   # choose the default sink

Software Volume Control for Optical Output

See step 2:

https://www.johannes-bauer.com/linux/dolby/