So I'm listening to the live recording on the "Pure Jazz" album.
The song is usually sung by a man, and Ella prefaces the song by mentioning this and talking to the audience. The band creeps in with rhythm section and a great walking bass line. At the beginning Ella stays true to the original melody--until the end of the first phrase.
The rhythm section keeps grooving while Ella, with such great ease, croons this fabulous song. All at once she says: "what's the next chorus to the song now, this is the one now, i don't know, but it was a swinging tune and it's a hit tune so we tried to do Mack the Knife....oh, Louis somethin', something about a cat, oh yeah darrin, he was spinning that trash, and that (something) dear he spins like a sailor, tell me tell me could that boy do something right....oh bobby darrin and louis armstrong they made a record oh they did....and now ella, ella and her fellas we're making a wreck, yes a wreck of Mack the Knife!" Now come the scat section (she's trying to sound like Louis) and of course the audience is dying laughing. She goes on to address the audience some more for the rest of the tune. It's hilarious! The whole while she's singing the melody and the rhythm section goes the whole while.
At the end when the applause picks up the rhythm section really takes it up, it's pretty great :)
Check it out, if nothing else-it will give you a chuckle, and we get to see how truly original and fearless Ella is.