The original Jan Lewan Orchestra members; Steve Saive Trumpet, Bob Lugiano Trumpet, Scott Shirk Saxophone, and Stephen Kaminski Saxophone, recorded the movie soundtrack at Avatar studios along with Jan Lewan who was coaching Jack Black on his vocals.
The real Jan Lewan, whose life this film is based, was in attendance at the Sundance Film Festival 2017 screening at the Eccles theatre. During the cast and crew Q&A, Jan joined Jack Black onstage to perform an a capella version of the Polka Rap.
Closing credits show some documents from original Jan Lewan's career. Among other, his Polish actors association card (dated 1964), signed by Henryk Szletynski, great Polish stage actor and the society's chairman. The card reveals also that Lewans real name is "Lewandowski".
The trivia item below may give away important plot points.
In the bus wreck towards the end of the film, the driver, 36-year-old Steve Saive, of Wilkes-Barre, apparently fell asleep at the wheel and lost control of the vehicle and actually killed two band members. Thomas Karas, 54, of New Windsor, N.Y., and John Stabinsky, 23, of Marlin, both died in the wreck, Karas played accordion and served as the orchestra's booking manager and Stabinsky played bass trombone and had been with the band for about a year. The orchestra was heading to Florida for a string of dates.