Where is the tuning knob on the machine?
closed as too localized by Mokubai, 8088, Diogo, Randolph West, Renan Aug 7 '12 at 3:50
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It has been many years since I saw a mechanical volume control on a laptop.
Nowday you usually control the sound with the keyboard. Look for a button Fn it's usually down to the left of the keyboard. This together with one of the function keys (F1-F12) on the top of your keyboard should do what you want.
On my netbook it's Fn+F7 mutes the sound, Fn+F8 volume down and Fn+F9 volume up.