speaker generally require a separate power source, make sure they are plugged in to an outlet (and turned on
Double check that they are connected to the computer correctly, it's usually color coded.
Open up the volume control in your OS and make sure it's turned up and not muted. Sometimes the volume gets muted for no apparent reason.
If you're still having problems, you can try reinstalling the audio drivers.
If that doesn't work come on back and describe the problem in detail.