You will need to know what sound device is installed on the machine...the information is available in Device Manager. If this doesn't help, we'll look at some registry settings.
Open Control Panel and double-click on the "Sounds and Audio Devices" icon.
Under the "Sounds" tab, select "Windows default" in the Sounds Scheme drop down menu.
Under the "Audio" tab, make sure that Your Sound Device is listed in the top two sections...if not, select it from the drop down menu.
Under the "Voice" tab, make sure the Your Sound Device is listed in both boxes...if not, select it.
Under the "Volume" tab, make sure there is a tick in the box to "Place volume icon in the taskbar".
Click "Apply" and then "Ok".
Now look down by the clock...is there a little speaker there? If yes, double-click on it and make sure there is not a tick in the box next to "Mute" and that the Volume sliders are all set to at least halfway.