New iMac mini DisplayPort to Panasonic Viera HDMI overrides Viera Link Soundbar

I've just bought a new iMac 27 inch. I've connected it to my Panasonic Viera TV via a Belkin Mini DisplayPort and Belkin HDMI 5m cable into the tv's HDMI port. The Viera Link Soundbar is connected via the ARC HDMI2 port. When the imac HDMI is connected to the TV it seems to override the ARC Viera Link and cuts out the normal TV out put even when the settings to the TV are switched of on the imac. It's really difficult to explain.