mid-2010 MacBook Pro to 5.1 sound

Hi

Connected my mid-2010 MBP to a 5.1 receiver via mini-display port-to-HDMI cable.

While the picture is fine, no sound is going through.

The System Preferences-Hardware-Sound doesn't offer an option to switch between outputs, as it shows Built-In Speakers option only.

what's wrong and how it can be solved.

Cheers