Do you have all Windows updates and updates to your system and any hardware devices by the manufacturers?
Please also see these:
If you are using WIN7 please see this: https://support.skype.com/en-us/faq/FA10330/Sound-
Note: the above link has 2 pages so please view them both.
Also, you can use the Skype client program window at anytime and do:
Hellp -> Call Quality Guide -> Sound Check Wizard
A new version has just been released August 23, 2011:
Please download from here and when prompted click the open or run button: http://skype.com/go/getskype-full
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