There's an episode where Peter reads a poem ("Who is this man called Rod Roddy?") and walks over to Rod's podium. When Peter finishes the poem, he and Rod sign off together. Immediately the fee plugs begin with Rod reading in a more calm, narrator-like voice. Now that I know those were pre-recorded, it makes sense how his voice changed instantly, especially since he and Peter were looking at the camera and a few audience members were frantically waving (one even bumped the lamp).