AirTunes Networking

Hello, I'm trying to connect a new AirPort Express to an existing network so that I can listen to Internet radio wirelessly over a hifi stereo. I have a USB cable that connects directly to the pre-amp. When I plug the other end into my Macbook Pro, the music plays through the speakers fine. When I connect the USB to the airport, which is connected wirelessly to an Extreme Base station, I get a message that the Express isn't connected to any speakers. How do I stream the music through the stereo without connecting the Express to a modem and to any speakers?

Thanks, Carlos.

Huh? You lost me with the USB statement. You need to use the 3.5-mm audio minijack for analog or optical digital sound. Never heard of anyone doing AirTunes with the USB interface?

OK, different approach. The only cord that attaches to the pre-amp is a USB. So, is there a USB to 3.5-mm audio minijack to plug into the Airport Express?