How do I get audio on my farenheit dvd-15?

We just bought a van with a farenheit dvd-15 installed (dvd player wired under passenger seat with flip down screen). The video is playing on the screen but no audio. It appears they set it up for wireless but the wireless headphones only pick up the radio. The only wires on the dvd is power coming in, video out and left/right audio coming out (also has a remote wire coming out). Shouldn't the audio be coming thru the speakers? Is there some magical setting I'm missing?

Thanks!