Check all of your audio cables. Make sure that they are plugged in correctly and that they are not loose. Also check to make sure that your sound card is tightly secured in the computer's slot.
On the other side, I would also check your computer's audio software and settings as popatim said as well. Also, how old is your computer and sound card, or is it an integrated sound source on your computer's motherboard or other on another chip?
It may also be your speaker system.