From what I've read, I don't think that's his claim at all. His claim is the audio recording will be made normally, however what will be heard on the tape (and as far as I can tell also witnessed live) will be a conversation with a supernatural entity.
The problem that I see is that there's no way to prove this entity is supernatural. It could be a real live human being cleverly hidden in the cemetery, or some sort of communication device with a human on the other end. There's no way to prove that the voice is coming from something (or someone) supernatural.
It seems his plan is to eliminate all normal ways sounds could end up on the tape. It seems a near impossible task to eliminate all of them as there are so many. From something as advanced as Hyper Sonic Sound to something as simple as two tin cans tied together with string. Given that the site hasn't been exactly secure, I'm uncertain how one would control for the possibility that a sound projection device of some sort hasn't already been embedded within the chair itself.
