Possibilities:
Audio channel is being overdriven.
Weak power supply
Electrostatic coupling between vid adn audio cards.
Incompatible drivers (sound, video or something DX9 related)
Audio channel is being overdriven.
Open the windows sound mixer (speaker symbol in the tay or go to control paenl sound advanced). Make sure none of the slider controls is above 50%. If they are the speaker amplifiers may be over driven.
The power supply.
How hefty is the supply (450-500W)? Does it have dual 12V rails. Is it a major name brand. Hoewmuch equipment do you have laoding it?
It could be electrostatic coupling between the cards.
I doubt it is electrostatic coupling becasue that would not be an issue in a movie, only during gaming. Movies put very little load on the video cared.
Get a peice of aluminum foil the size of the audio card, sandwich the foil between 2 peices of thin card board (serial box, cookie box, etc) with a wire coming out. Build Instructions - Lay cardboard 1 down, lay the foil on top, run a thin strand of wire from side of the foil to the other with 5 inches extending out past the edge. Tape the wire down against the foil with scotch tape. Lay the other cardboard on top. Lastly Run a strip of scotch tape around the edges so no metal can be touched except the wire coming out. Slip your brand new Faraday shield bewteen the cards, preferably scotch tapped to the sound card so it won't block the video card fan. Take the wire end and run it to the backpanel screw for the sound card and wrap it around the screw once before you tighten the screw down. Cut off the excess wire.
Driver issues is always a possibility.
Unlikely with these symptoms but... Make sure you have the latest audio card drivers and video card drivers. Check the audio and video card websites and read their FAQ's email their customer support.