Have you installed the latest version?
Before trying to isolate an issue, it's always best that one is running the latest Skype release. Which may in itself resolve the issue.
A new version has just been released August 02, 2011:
Please click the "Get Skype" link at the top of this page and download the normal version of Skype, NOT the beta version to get this new version.
This new version is 18.104.22.168 to check what version you have, using the main Skype client window please do:
Help -> About Skype
My skype says there is a problem with playback device, what does that mean how do I fix it??? PLEASE HELP
What settings do you see for Speakers in Tools -> Options -> Audio settings? Is there more than one option avaiable in the selection box? Do you hear any sound when you press the test button next to this selection box?
Just posting this in case anyone else ever has this issue and runs into this post while searching...
Easiest way to fix this is to right click on your sound icon on your taskbar and select Playback Devices. Then, right click on the active speaker device and and click Test Sound. If you were not able to successfully hear a sound or received an error, right click on the device again and click Disable, Then, by the same means, re-enable it. Test. You should hear sound at this point, Skype should now be working.