Need to know if this is a pre-built (make and model of the computer) or a custom build (make and model of the motherboard). If the sound device is not integrated (a separate card in a PCI or PCIx slot), then we'll need the make and model of that too. If it is part of the motherboard (integrated) then just the model of the motherboard or computer will suffice.
If you're not sure which is which, an integrated audio chipset will have the jacks next to the USB, network, PS/2 and other ports. A separate card will be located in one of the expansion slots away from the other ports.
If you haven't already done so, check that the speakers are working properly by testing them with another computer or device such as an MP3 player. Also, test another set of speakers or even headphones with the computer.
Additionally, make sure the speakers are in the correct jack (usually colored greed or indicated by a speaker or headphone icon). Gently rotate the plug in the jack to make sure it's making contact. Gently rub down the wire to check for a damaged cable.