A participant is someone who 'calls in' to the show using an audio chat program. Playing back the second file I heard that the participant's part was missing. This is somewhat logical because what the participant says should go to the other side of the chat and should not be played back locally. However, I was routing all audio from my 'studio' back to the participant. This means that iChat probably has a mechanism to filter out anything that originates locally.
More on the technical details of this setup in a later post.