If you have multiple sound cards in your pc and want to set one as the default card you can do this like this.
With the command:
lsmod | grep snd
we can find out which sound drivers are loaded. On my pc it is sndhdaintel and snd_virtuoso.
If you want set the priority of the virtuoso (Xonar DX) higher you edit the file:
I set the index of the viruoso device to zero so that it is the default device:
options snd_virtuoso index=0 options snd_hda_intel index=1