I'm having a similar problem.Got a Time Capsule based network running 7.6.3 (recently updated). Previously, I also had two 1st gen Airport Expresses extending this and two 2nd gen Apple TVs for Media in different rooms. The Apple TVs were connected back to the TC via ethernet but would also play content from iPhone/iPad over Airplay. Happy Days.
Now that I have upgraded to the latest Airport Expresses (again two, to take advantage of the 5 gHZ network) my Apple TVs only work over ethernet. So they will stil pull content from the ethernet network but they only connect to my WiFi (and Airplay) if the ethernet cables are unplugged. If I unplug the cables, they connect to airplay, as soon as I re-connect ethernet, they drop wifi and lose the abuility to stream Airplay. Seems they have lost the ability to simultaneously be part of a wired and wireless network.
Any ideas?