- Make sure your digital output in your sound card software is turned off
- Try a different power source
- Try a different audio source
If the speakers do not hiss when the computer is turned off, then the hissing is coming from your soundcard.
Try muting devices you are not using in the volume control panel, such as Line In, CD Audio, and Mic Input. Also try moving your soundcard to another slot of your motherboard, away from other devices. If the speakers do hiss when the computer is off.