Latitude E6530 - No sound through DisplayPort on PR03X docking station

I have a new E6530, which I am using with a docking station (PR03X I think). I used the same docking station for my M6500 and was able to use audio through the Dell SP2309W display HDMI , through to its soundbar. With the E6530 however, the video is fine through the same dock-station displayport adaptor and cable but I can't get any sound. When I look in the NVidia Control Panel under "Digital Audio" it shows DisplayPort (1), DisplayPort (2) and HDMI as having "No audio capable display available". If I connect the monitor directly to the computer's HDMI port it's fine - sound goes through the HDMI cable to the display to the soundbar.

Why won't my E6530 send sound through the same docking station displayport connector which the M6500 had no problems with?

PS - I have updated my BIOS and NVidia drivers, I'm using Win7 64 bit. The video and audio sources for the SP2309W is set to HDMI.

Thanks.

Muffy