I recently attached an Apple TV to project to my board wirelessly. The HDMI cable goes to the projector. However, I am not getting any sound from the Apple TV. Sound only comes from the USB cable on my computer.
If the projector being used with this board has an audio output (some Promethean-branded Projectors will have a 3.5mm port for that output), you will need to have a cable routed from that output to one of the input on the amplifier on the side of that board or into whatever audio system is in use in the classroom.
If that output is not present on the projector, then you will need to route that audio from the AppleTV directly to your audio system or the inputs on the Board’s Amplifier, using any necessary adapters.