My case: I have an on-board sound card that I use for everything except for Skype, where I use my Logitech ClearChat Wireless USB Headset. It is easy in Skype to select a different sound device. So I needed to set my on-board sound card as the default output for everything else.

If they both use a different kernel module you can easily make sure the right one is the default card.

Find out what module is being used for your soundcards:

cat /proc/asound/modules

On my computer this gave

0 snd_usb_audio
1 snd_intel8x0


Sound card 0 is the default one (snd_usb_audio in this case). This is the module’s name.

You can’t assign card 0 because whichever card get’s loaded first get # 0. So, you have to assign # 1 to the one you don’t want as the default card. This can be done by adding one simple line to /etc/modprobe.d/sound (create it if it doesn’t exist):

options name_of_offending_module index=1

So, in my case this would be

options snd_usb_audio index=1

Reboot and try

via HOWTO make one soundcard default.

http://aspn.activestate.com/ASPN/Downloads/ActivePerl/PPM/

Alternate packages repositories:
http://cpan.uwinnipeg.ca/PPMPackages/10xx/