if You have a GPU and it has HDMI, use That to run from PC to TV, that way the GPU will use its audio , and it will have to route through the TV, then when You change the TV source from HDMI 1 (PC) to HDMI 2 (TV) the TV will handle all of the audio itself, and the PC will only output audio from that HDMI cable when Your using GPU audio, so it would be impossible to have sound coming out of the PC speakers after doing this.
it sounds to me like You have onboard audio running using 3.5 jacks , or whatever your motherboard has for audio output, using the GPU will remove the option for that extra level of complication.