I have a similar question to this one but I'm on Mac OSX. I currently have a laptop, a second monitor (via HDMI), and a third monitor (via HDMI/thunderbolt adaptor). If I have sound playing, I only want it to come through my headphones. However, sometimes the sound will play through one of the monitors if the window playing the sound is on that monitor.
I tried going to "Sound" in System Preferences, but for both HDMI connections it says "The selected device has no output controls."
How can I change it so that audio does not get output via these HDMI cables? Alternatively, how can I change it so that if the headphones are plugged in, sound can only come out of the headphones?