Wii U Audio Problem

I use my HD monitor to play games through its HDMI port, but because it doesn't have built-in speakers, I usually output audio through the basic AV (the red and white tips) cables simultaneously on my external speakers. I can't figure out how to get sound to play through Wii U mode, but weirdly, I get sound when I load up the basic "Wii mode."

This basically means if I want to play the Wii U, I get video but no sound. Is there a step I'm missing here?

Any help would be GREATLY appreciated, since it's no fun having sound only come through the GamePad, especially during multiplayer.

Thanks!