Disabling HDMI Audio and using Digital Optical only?

I am not even sure if this is plausible with aTV Flash as I know this is how the hardware is meant to perform. However it is very annoying having to turn the volume down on the TV when on the aTV input then switch it back on afterwards when I am done. (Unfortunately my TV doesn’t allow me to mute the speakers depending on the input automatically).

Therefore I am turning to you guys to see how plausible it is to do this. I understand that aTV Flash is only software but maybe there is something that can be set that will do so.