Do not hear sound on Skype

Hello to all member of techyv.com,
I am a new user of this site. It is very help full site. Always it's help us by problem solving. Now I want to say my personal problem. Please anybody help me. Online earning is my profession. For this reason I have to use many messenger like Skype, yahoo messenger, Gtalk messenger, Facebook chat, meebo etc. But recently I have bought a notebook (second hand). That's all are ok. But when I want to use Skype, I can't hear anything, why this problem or how can I remove this problem, please help me anybody.
N. B: I install Skype latest version & Notebook of HP.
