ALSA

https://wiki.archlinux.org/index.php/Advanced_Linux_Sound_Architecture

Setting Default Sound Device

For example, setting the SP/DIF Digital Output as the default device...

aplay -l

**** List of PLAYBACK Hardware Devices ****
card 0: SB [HDA ATI SB], device 0: VT1708S Analog [VT1708S Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: SB [HDA ATI SB], device 1: VT1708S Digital [VT1708S Digital]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: SB [HDA ATI SB], device 2: VT1708S HP [VT1708S HP]
  Subdevices: 1/1
  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

Now, create and edit the ~/.asoundrc file with the following lines...

defaults.pcm.card 0
defaults.pcm.device 1
defaults.ctl.card 0

Then, test...

aplay /usr/share/sounds/alsa/Front_Center.wav