I tried this just now and it didn't work for me.
I started the car, inserted the dvd, and started playing the video.
When I pressed the voice button on the steering wheel (with the microphone icon, on the left under the d-pad) the video paused with the mute icon on the upper left of the screen.
When I pressed the sound button on the RNS-510, the sound options menu appeared (blocking the video).
I wasn't able to get the video to keep playing while driving, using any combination of these two buttons, holding both or pressing and letting go.
Unfortunately I don't have my vagcom yet, so I don't know the firmware version on my RNS-510, and I don't want to pull it out just yet. I received my 2010 JSW on the last day of June (a month ago).
Am I missing a step? Let us know wallyballz.