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ALSA (Advanced Linux Sound Architecture)

ALSA is more modern than OSS. Currently, Skype does not natively support ALSA, so you will need to install ALSA OSS emulation layer in order to use ALSA devices in Skype.

You should have snd-pcm-oss and snd-mixer-oss modules in your kernel, either loaded separately or compiled in as part of the kernel.

You can check for presence of these modules by issuing (as root user) the following command:

lsmod

If you don't have these modules, try to load them. Type (again as root user):

modprobe snd-pcm-oss
modprobe snd-mixer-oss

http://www.skype.com/help/guides/soundsetup_linux.html