Auto in what sense?
Perhaps when you open some media on the main monitor and slide it over to the TV, you'd like the audio source to follow the image?
Is that what you mean?
Allow me to tell you the little that I know about it.
I generally run on two monitors, but when I disable one of them and make my TV the second monitor, my audio does automatically change to the TV, but only because of the graphics card, not Windows.
In fact, we recently had a thread talking about it, as others had it working that way, while mine wouldn't, but once I understood that it depended on the graphics card that supplies the HDMI to my TV, I got some other drivers and that solved the problem.
In short, there is no auto function to determine where the audio will come from, depending on which monitor you're viewing it on.
It's a simple case of enabling the monitor, in this case, our TV's, that are connected by HDMI, and the GPU makes the audio change at that moment, regardless of any media playing on either monitor.
I hope I explained that well enough to be understood.
Lastly, there are surely some settings that control how that all works, but if you're looking for one that changes the audio as it follows the image, I suspect you won't find one.
Then again, I'm no expert on the matter.
If you're on this ignore list, vivahate, sd44, cav, charles052, frogstar, godfather927 don't bother quoting me