There could be a couple of things causing this.  Do you have ALSA's OSS
Emulation properly configured.  If you do, you should be able to hear sound
when you #echo /dev/rand > /dev/dsp .  If /dev/dsp isn't working, please
refer to the Gentoo Wiki article on setting up ALSA properly for OSS
Emulation.  http://gentoo-wiki.com/HOWTO_ALSA_Complete_%28includes_dmix%29

You may also want to check on which version of ALSA you're using, because
prior to ~0.9, dmix wasn't automatically enabled (software mixing) and if
your sound card doesn't support hardware mixing, other devices will tie up
/dev/dsp and not allow Skype access.  Skype is very stingy when it comes to
sharing /dev/dsp.

Let me know what you come up with.

Jason Weisberger