Do you have another set of speakers you can try, check all the connections, both outside and inside the case, also, check that there is nothing muted, and check the volume control, i know, your thinking volume control ?, is he mad, but you would be surprised at the things we come across here.
Is the sound card on the Motherboard, or on a separate card ?. If it is a PCI card, try re-seating it.
How do the speakers connect to the PC ?.
Has the PC been moved recently ?.
Have you installed/un-installed any hardware/software recently ?.
These are all things you have probably done, but just recheck everything.
If you have checked everything, and its still not working, then you need to consider the sound card itself.