Because HDMI is a video standard first, audio second.
I think what you're doing is the best solution. Thing is, I'd move the secondary from the top right to the lower left corner. A lot of applications look for the top right display (0,0 pixel coordinates) so having it up there by itself can create problems. If there's nothing happening on that virtual display, it's not going to draw much power beyond refreshing the screen. To reduce that, turn the Hz down as low as it will go.
Firmware update for the TV might help:
https://www.samsung.com/au/support/model/UA40KU6000WXXY
I would say install the drivers for that TV but drivers apparently don't exist for it...however, there is this (make sure to select the PC option for the HDMI in you're using because it turns off the scalers)...
https://www.samsung.com/au/support/skp/faq/1049290
Also turn on "Game Mode" if you haven't already:
https://www.samsung.com/au/support/skp/faq/1049274