The USB port on the Airport Extreme and the Airport Express is the same. On both of them, the USB connection is for a printer or USB drive only. There is no audio output.
Here is the language from the specifications page for the Airport Extreme on the Apple Web site, "USB 2.0 port for connecting a USB printer or USB external hard drive."
Here is the language from the specifications page for the Airport Extreme on the Apple Web site, "USB 2.0 port for connecting a USB printer or USB external hard drive."