[Ira Herman]

I had to reply here because the Internet needs more info than checking a page in the manual. Fortunately for me, I have the manual so that was actually very helpful. Here’s what to do:

To turn the volume up or down in video mode:
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hit the Q button
hold the RATE button and press up or down on the multi directional controller.

This turns the volume up or down — there is no visual display of headphone level, you can hear the difference though as you do it.

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Hope that helps future people who find this on a google search!