It would be best if you found a way to connect your AirPort Express directly to your home network, and disable NAT and DHCP service in the AirPort Express. It can just be a simple bridge or a client device since you've already got a router doing NAT and DHCP service for your network.
If you really wanted to make it work with the Mac Pro being the way the AirPort Express connects to the network, you could probably make it work by using Internet Sharing on the Mac Pro to make it act as a Wi-Fi AP (and home gateway router), but then do something to block it from being a DHCP server on its wireless interface (kill bootpd
, or use ipfw
or pf
to block bootpd
's packets from getting out), and then create a bridge device and add the Ethernet and the Wi-Fi interface to the bridge device. Then reconfigure the AirPort Express to join the Mac Pro's Wi-Fi network as a client.
You need to be bridging the AirPort Express's traffic onto your home network, otherwise iTunes (or other AirTunes/AirPlay clients) won't see the Bonjour service advertisements, and thus won't know that an AirTunes/AirPlay speaker device is on the network. Also, AirTunes (at least used to) have a restriction where it would only play to devices on the same IP subnet. So putting your AirPort Express on a different subnet than your home network probably won't work.
The "needs to be on same IP subnet" issue is also why I suggest you make sure to block your Mac Pro from acting as a DHCP server (which is something Internet Sharing automatically turns on). Because if your AirPort Express gets a DHCP lease from your Mac Pro, it won't be on the same IP subnet as the rest of your network, so AirTunes will probably balk.
One other possibility to avoid running Internet Sharing is that you could try putting your Mac Pro's Wi-Fi interface into IBSS (a.k.a. "ad hoc", "computer-to-computer", etc.) mode, but I don't think AirPort Expresses support joining IBSS networks. So you need to put your Mac Pro's Wi-Fi interface into real Access Point (AP) mode, which is what Internet Sharing does. I don't know of an easier way of putting the Wi-Fi interface into AP mode other than starting up Internet Sharing. I don't think you can trigger that mode with ifconfig
, airport
, airportd
, networksetup
, or anything else that I know of.