In Mama Roux by Dr. John, there's an instrumental break near the end of the song (around 2:18), but you can hear Dr. John singing another verse under the music. What is this verse, and why is it missing?
It is not a missing verse. It is Mac singing off mic during the instrumental break, most likely to help himself keep time for when his lead vocals return at the end of the break. It was probably not intended to be part of the recording.
The treatment of the song varies during live performances available on YouTube but none of them introduce a verse that may have been cut from the recording.