If your computer came with a motherboard disc the sound drivers could be on it
click start control panel administrive tools computer management device manager scroll to sound,video and game controllers you might see a yellow question exclamation mark? ! or a red X Right click to reinstall drivers

click start control panel sounds and audio,volume,advanced,speakers make sure you have selected the
(appropriate speakers) also audio you will find 3 drop down lists make sure you have the correct settings there

click start control panel sounds and audio devices select advanced there might be a tick in the mute box untick it