1. Install pavucontrol if it’s not already installed
2. Start pavucontrol
3. Play a sound in the application where you need to change the default sound device.
4. While the sound is playing, switch back to pavucontrol and change the sound device.
5. that should be it.

I needed to set my Logitech Clearchat Wireless Headphones as the default sound device for Skype 2.1.0.47 beta in Ubuntu Karmic Alpha (pavucontrol was not installed).

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.