OK, first you have to isolate the problem, if it is the computer or the speakers.
Try a set of headphones instead if speaker. If the headphone does the samething than you know that it is the computer.
If the head phones works just fine, than it is the speakers.
If speaker problem: Look if there is any lose connection.
If it is the computer: Go to sound properties and look if everything is set right. Try to connect the speaker on the front of the computer if you have a front port. Uninstall and resintall driver. Etc