Cavechat.org

Has anyone encounterd hearing someone shouting from a long distance away sounding like a squeky old crystal radio. We were mapping this small tunnel that was about 30" tall and 4' wide. I took a shot with the disto and it was 44 meters long. I heard this mosquitoe in my ear. There is was again. I yelled out then my buddy yelled back. I could just about understand him. I yelled back the measurements. Then when I got to him I said he sounded like a mosquitoe. He said he was just about to say the same thing to me. Very weird.

Is there a helium pocket in the cave?

No, :) Our voices were totaly normal once I got closer to him.

That can only mean one thing, The Acoustic Cave Demon

You might look into the topic of accoustic resonance and damping. Here is a link to get you started: http://en.wikipedia.org/wiki/Resonance

I've experienced this before. You must have just been in that acoustic sweet spot for that to occur.