Finally after trying to uninstall/reinstall (no luck!)
If found a thread about IPv6.
I followed the same directions on Windows 7 64 bit and it worked.
Disable IPv6 on _all_ of your adapters. That includes Virtual Machines (VMWare... yes those too), even wireless.
Close iTunes. Start iTunes. Works. Airplay Icon is back.
It looks like All of the AirPlay devices may have to be re-setup. They won't auto configure.. yet
Even though this is for the Mac... I believe they are onto something.
For all Windows 7 users, here is a link that will run a script to disable IPv6 restoring airplay in itunes.
Run fix 50409. It will ask you to restart and airplay will be restored.
Ran the link and rebooted but no luck. Still no airplay icon available in itunes(10.7.0.21) win 7, 64 bit pro for me. Actually nothing Apple wise seems to ever work for me. I have ATV 3, ipad 4 and mini and a many iphones 3,4s, and 5 models and nothing ever fully works as described by Apple. My home network works perfect for eveything but Crapple!
I may have the solution. I have tried everything I have found on the web, to no result. No airplay icon on the full screen of Itunes 11 and could only change the volume with that of the mini player.
Then I have installed all possible updates through windows update and it appeared.
Hope this helps.
Open control panel.
Choose Network and internet
Select the VIEW NETWORK STATUS TASKS under NETWORK and SHARING CENTER section
Choose wireless Network Connection or something similiar. If connected with ethernet then select it and press right button.
Click the properties button
Find Internet ''PROTOCOL VERSION 6(TCP/IPv6)'' anduncheck it, then click OK.
Restart your PC and try.
Reason For Problem
When u will use ipV6 , you have to setup your router and lan/wifi in ipv6 mode. Then bejour pass sound through ipv6 but when you use 2 mode ( ipv4 and ipv6 ) together benjour gets confuse some time it pass through in ipv6 and sometime ipv4 .if your router setup is ipv4 then, when banjour pass sound through ipv6 you will not see airplay button in itunes.
Hope it's clear to everyone .