Audio problem: In the scene at the Palace Hotel Ballroom, when the Blues Brothers are singing their 2nd tune (Sweet Home Chicago), you can hear both Jake and Elwood on the soundtrack but Elwood is dancing down the middle aisle without even a microphone in his hand and his mouth is not moving. There is also a shot during the same song where Jake's singing and pauses do not line up with what we see visually. When he sings "baby don'cha you want to go-oh - -" we see his mouth stop moving, then when he pauses, his mouth is still singing. As a matter of fact, the entire number has mistakes like this and mistakes of continuity. (01:54:20)
Audio problem: When the band is playing at Ray's music store, Murph is playing a tambourine totally out of synch with the music. (01:09:55)
Audio problem: In the soul restaurant, when the Aretha Franklin song "Think" is about to end, watch the mouth of the fat lady behind her and you will see that her lips don't match her song. (01:05:35)
Audio problem: Near the end, when the officers are getting out of the van, they are shouting "Hut! Hut! Hut!" but their lips aren't moving. (02:13:45)
Answer: It was a double, and the "making of" documentary on the DVD shows this.
David Mercier
To be fair, John did do a lot of flips and dancing as Jake; just not all of them in the movie.