Mac mini running Plex and no sound coming through the amp.

I've got a Mac mini OS X 10.6.8  and I downloaded the Plex program to play movies and tv shows. I can't get any sound through the amp while watching through Plex. The Mac mini and amp are connected via HDMI cable and I know it's been set up right because I can run iTunes through the amp with no problems.
Anyone have a solution to get sound through the amp?

For sound, try the audio output port and connecting to the RCA  audio input ports on your Pioneer receiver.  Use something like this: