As you can have sound from other sources, it's not a hardware problem, at least not on your side.
Double check the settings in the Skype application. Maybe the sound is mutted.
Maybe you have no sound because the other person effectively don't send any...
Have the other person double check the settings in the Skype application.
Ask him to make sure that his microphone is properly connected and enabled.
Double click the sound icon in the notification area, un-mute any mutted controll. Set them all to at least 50%.
Electro, July 2011