Surround Sound Volume is unbalanced

I have a contour minibox --> HDMI Denon AVR-391 Receiver --> HDMI Samsung 55' TV.

This happens on the ABC channel with the show "The Voice".

As soon as the artist begins to sing, the sound on the front and center channels will drop to nearly 0. The back channels remain at a constant level. When the artist finishes,  the volume on the center and front channels go back to normal.  I have tried various settings on the receiver but it happens with all of them.

I have 2 other contour miniboxes connected directly to TVs (no surround sound receiver) that have no issues with sound.

My guess is that there is something with the signal coming out of the contour minibox box controlling the 5.1 surround sound.