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I haven't heard about using HDMI for audio only. But it would be helpful to know why you're doing it, because there are alternative ways to get audio between your Mac and some other place (for example, using a 3.5mm-to-3.5mm cable). Or, if your headphone jack is already occupied, use a 3.5mm splitter).
 
OSX won't let you turn off a display that you plugged in. All you can do is mirror it to one of the screens or extended display.

Windows can do this, though. You can also get an apple tv and wirelessly connect to audio over wifi, or use 3.5mm jack/optical in.
 
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