Pitch Perfect

At a 2011 national a cappella competition, Barden University's all-female a cappella group, the Barden Bellas, perform well until Aubrey Posen (Anna Camp) vomits on stage during her solo. They are publicly humiliated, losing the competition. Four months later, newly arrived Barden freshman Beca Mitchell (Anna Kendrick) has no desire to attend college, but she is forced to do so by her father (John Benjamin Hickey), a professor at the university, with whom Beca has a strained relationship. Wishing she could instead pursue a career as a music producer in Los Angeles, Beca spends her time making mash-up mixes of songs and takes up an internship at the school radio station, where she gets to know fellow freshman Jesse Swanson (Skylar Astin).
When Beca's father discovers she has not been attending classes, he comes to her with a proposition: if she joins a club and still does not want to remain at college after one year, he will allow her to leave college, and he will pay for Beca to move to Los Angeles to pursue her music career. After some persuasion from Chloe Beale (Brittany Snow) the co-leader of the Bellas along with Aubrey, she reluctantly joins the Barden Bellas. Aubrey does not initially approve of recruiting Beca, but yields nonetheless due to a desperate need for members and Beca's talent. Beca joins alongside a group of other women who do not fit the Bellas' typical mold: Cynthia-Rose (Ester Dean), Stacie Conrad (Alexis Knapp), Lilly Onakuramara (Hana Mae Lee), and Fat Amy (Rebel Wilson). Jesse is also revealed to have joined the Bellas' all-male rival group, the Treblemakers, but his roommate Benji was rejected after being called "weird" by Bumper.
The Bellas participate in the southeastern regional competition at Carolina University where, at Aubrey's insistence, they perform the same medley that helped the Bellas advance to the finals the previous year. In spite of their old set list, the group manages to place second, which sends them to the semi-finals. Unfortunately, disaster strikes for Beca when the Treblemakers cause an uproar with the Bellas after the competition, leading to her accidentally smashing a window with the trophy, which alerts a police officer standing outside who then arrests her. Jesse later bails Beca out, but makes matters worse by getting her father involved in the situation, who is furious and tells Beca that he will no longer pay for her to move to Los Angeles because of the incident. This infuriates Beca, which turns her budding friendship with Jesse sour. In the meantime, Beca suggests to the Bellas to try to be more daring, but Aubrey insists that they will win with the repertoire the group has always used in the past.
At the semi-finals, Beca provides an impromptu back-up of one of her favorite songs into the "traditional" song on the Bellas' set list in an attempt to reinvigorate the crowd after they seemed to have lost interest. Even though the audience seemed to enjoy Beca's remix, the Bellas come in third in the competition behind the Treblemakers and the Footnotes. Aubrey becomes furious with Beca, who in turn quits. But things get better for the Bellas when the Footnotes are later disqualified from the national competition because the lead singer was actually still in high school, calling the Bellas to take part instead. The Bellas re-form after spring break, with the notable exception of Beca. During rehearsals, Chloe stands up against Aubrey's stubbornness. The group starts to fall apart, which sparks a fight. Meanwhile, Beca tries to apologize to Jesse, which he declines. Beca then apologizes to the Bellas for her actions during the semi-finals and asks to have another chance, which Aubrey eventually allows. After having a group heart-to-heart conversation, the Bellas decide to adopt Beca's more modern and original music style. Meanwhile, group leader Bumper (Adam DeVine) leaves the Treblemakers after being offered a job as a back-up singer for John Mayer. With Bumper gone, Jesse persuades the Trebles to let his previously rejected roommate Benji (Ben Platt) join the group in Bumper's place.
At the a cappella national competition, the Bellas sing a piece arranged by Beca, which includes "Don't You (Forget About Me)" a song featured in The Breakfast Club, one of Jesse's favorite movies. This acts as a more effective apology from Beca, and after the performance, she and Jesse kiss. The Bellas win the national competition, and six months later auditions for new members take place.

Question: Who provides an impromptu back-up of one of her favorite songs into the "traditional" song?
Answer: Beca