I have the AirPlay enabled B&W Zeppelin Air all set up correctly on my local WiFi. However, quite often when waking my iMac from sleep it will have disappeared from the list of AirPlay devices in iTunes to stream to.
I am not sure what the problem is. The speaker still shows up on all my other iDevices, and in iTunes I still get my AppleTV in the list of devices. I also can ping the IP address of the speaker from the computer without a problem.
So far the only solution for me has been restarting the computer. Just restarting iTunes does not do the trick. Disabling and re-enabling my network interface (it is connected through cable) also seems to work. However I am looking for a more elegant or permanent solution.