After collaborating with the Cat, the Dumb Mouse is holding session with the other mice, and they are singing a song to the tune of Blues in the Night (My Mama Done Tol' Me). The song was first recorded by Artie Shaw and His Orchestra on 2 September 1941. The song subsequently was recorded by many others, including Woody Herman and His Orchestra, Big Joe Turner, and Cab Calloway and His Cotton Club Orchestra.
(at around 2 mins) After the Cat finishes whispering in the Dumb Mouse's ear, the Cat's normally-pointed teeth change to being squared-off and buck-toothed as was used stereotypically to represent the Japanese.
The title Fifth-Column is a term for someone who is part of an attempt to undermine an oppressive rule.
Vitaphone production reel #913A.