Q: I have two AirPort Express units connected to individual stereo systems, and I play music from my iTunes Library through them. I have recently noticed that I am now able to play music on “Multiple Speakers,” which is really neat, except that I get an error message reporting that one of my AirPort Expresses does not support playback as part of a group. Why not?
A: There are two potential issues that users often encounter when attempting to play music to multiple speakers simultaneously:
First (and most likely your problem), you’ll need to make sure that each Airport Express unit has been upgraded to the latest firmware (v6.3 or later). You can check this using Airport Admin Utility, and if you need an upgrade, you can obtain it from Apple’s website here: (Mac, PC). You’ll also need iTunes 6.0.2 or later.
The second potential issue that often hinders users from using this feature is that it requires a certain kind of network traffic often blocked by many firewalls. This “Incoming UDP traffic” needs to be allowed to pass into the computer you’re running iTunes from. In your firewall configuration settings, either set it to allow all UDP traffic, or simply open UDP traffic on ports 6001 and 6002.