I dont think there is a way to. Cant control hard wiring on a speaker with windows.The aux doesn't even involve the PC in anyway. its a direct "plug it in, the speakers play it".As far as windows knows, there is a sound output jack it can send sound to, completely separate and unknown is the AUX input to the speakers. Windows/ your sound card has no idea what is plugged into it or even if the sound its sending out the green jack is being used or going to a dead end loop while the AUX is being feed into the speakers.
And if the AUX jack works like headphone jacks on some stereos no way to do it without unplugging it. if its the simple "plugging in a source separates the 2 contacts from the other connection" its a mechanical thing, so even a switch wired between the aux plug and its source would do no good. As the pressure and size of the plug is what separates the contacts, and the plug fits between them to make it the 'new' input.
Basically you have a set of speakers with 2 inputs, with a mechanical toggle switch built into the AUX plug that says "play this input, or this one"
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Disclaimer: Post made by me are of my own creation. A delusional mind relayed in text form.