I believe the input for a 7.1 surround sound system consists only of a left and right channel. The phase and amplitude between the same frequencies on both channels determines where the sound is played in final in the 7.1 system.
For example same phase same amplitude will be played in the front center speaker.
You can probably easily build a manipulate with a single frequency and a couple of controls (phase and left/right amplitude) to play a single frequency on any of the 7.1 speakers.
(I do not have a surround sound set up here)
Please share with us if you are successful.