When using an AppleTV to wirelessly present your slides via AirPlay, you need to split the audio from the HDMI output of the AppleTV, so you can run the video to your Projector and a separate audio line to your soundboard. Here's some hardware you can use.
HDMI to HDMI with 3.5mm Audio Splitter
This HDMI2HDMI has an HDMI input to one stereo 3.5mm headphone jack output and one SPDIF output. We've found it an Amazon for ~$40.
HDMI to HDMI Audio RCA Splitter
This splitter has one HDMI Input to One HDMI Output, One SPDIF Audio Output and One RCA R/L Stereo Audio Output for ~$32 on Amazon.
HDMI to VGA and Audio Jack Splitter
This splitter is the Kanex ATV Pro HDMI to VGA Adapter with Audio Support. It's ~$49.95 and available through Apple.