speaker hiss?

Maybe the speakers are set to a volume that is to high.
Maybe there is something deffective in the speakers.
Maybe there is a problem with your sound card.

Try reducing the volume on the speakers, but increase the output volume on the computer side.
Click the sound volume and set it at least to 75%.
Double click the sound volume.
Set the main and wave sliders around 75% to 100%.

Set the speakers volume around 20% to 40%.
(mines have noise if I set the volume over about 40%)

If you don't have a notable improvement, try your speakers with another computer.

If they still hiss, it means that their power supply or the internal amplificator is deffective. Get new ones as it should be cheaper than changing the power supply or amp.
It's probably not possible to get the needed replacement parts anyway. It's also possible that you cam't open them without causing serious damage to the speakers' case.

If the sound is correct on another computer:
Make sure that your sound card's drivers are properly installed.
Also make sure that you have all the correct codecs.

It the sound is still bad after reinstalling the drivers and codecs and making sure that everything is properly configured:
Your sound card seems to be deffective.
Get a new one.

Electro
May 2011