On the video you provided, it sounds like static interference and not true "feedback". So it could be electrical interference from the computers you're using and as such, have you tried using shielded speaker wires?

Have you made sure all microphones are turned off when the speakers are on? Cordless and wireless telephones can cause such interference. Have you removed them all from the speaker proximity?

Have you updated the sound card's driver? If not, visit the sound card manufacturer's website and download/install the latest driver for your card. The driver should also be available at your computer manufacturer's website.

Hope this helps.

Grif