AirPlay is not restricted to wireless only. It can also work over Ethernet. Basically as long as there is a connection (on the same subnet) between the iTunes host and the AirPlay "speaker" streaming should take place without any issues.
FWIW, My Win8 PC is connected to a Time Capsule via Ethernet. In turn my Apple TV is connected to the TC's wireless network. I have no trouble streaming from the PC to the Apple TV in this configuration.