Audio setup for Macbook and XBox?
You are right that the AirPort Express would work for the iPod and Macs. It won't, however, work with the Xbox. For that, I recommend getting a little RCA switch and RCA-minijack adapter. Using the adapter, plug the AirPort's audio out into one of the input ports of the switch. Plug the Xbox another input port. Plug the speakers into the switch's output end.

When the switch is set to the AirPort's input, you can AirPlay music. When the switch is set to the Xbox's input, you'll hear sound from your Xbox.