Apple TV Skips Sound for Video Playback

Hi,

We are a school using a setup of Apple TV attached to a projector through HDMI cable and a set of mounted speakers connected directly the projector in each classroom. Macbook Pro/Air uses the school's wireless network to connect to Apple TV to present their screens which includes videos. There has been several instances where video skips sound. Apple TV is 3rd generation (A1469) and the OS is 7.2.

We have tried the following:

1. Changed Apple TV
2. Changed HDMI cable
3. Tried a different computer
4. Also changed the wireless access point

Any suggestions would be greatly appreciated.

Thank you,

Abeda.