Because if you dont move the mouse the cpu speed is lower and many motherboards have noise in lower voltage state(quick changes of low voltage).So you dont have coil whine as understand it but noise in 5 rail. You can bypass it if your speaker have external power and not from usb. You can try it.
I hope you don't waste much money on these hardware swaps, I mean you've changed GPU, PSU and audio speakers.At least you found out the cultprit - or a good lead, hopefully this will fix it for you.
I was able to return the GPU (yeah Amazon) though I would have liked to keep it but couldn't justify the $159. I'm not a gamer and the 750 works great for me.
I am keeping the PSU though. $59 I can sluff by my wife(she really doesn't care but we like to joke about it).
Put a new set of speakers in just a few minutes ago and the whine is finally gone, yeah!!!! I can hear my Tinnitus again
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