video monitor with intensity shuttle issue

hi there

just plugged in my new thunderbolt intensity shuttle and connected a monitor to it with hdmi, and it has cancelled the audio on my headphones attached to the mac

i'm now having to keep swapping my headphones over from the monitor headphone socket and back which is a bit of a pain

does anyone know if there's a box i should be ticking to just send the video and not the audio?

any help, as usual, most appreciated

P