Whiplash is a fairly straightforward psychological drama, but one question is left unanswered when the film ends, as it's never revealed who took Tanner's (Nate Lang) music folder. The 2014 movie follows Andrew (Miles Teller), a young and competitive drummer studying at the prestigious music academy, Shaffer Conservatory. But he quickly learns that it's going to be a difficult course when he meets his terrifying and emotionally manipulative music instructor, Terence Fletcher (J.K. Simmons). The instructor takes pleasure in making his students cry, laughing at their personal lives, and criticizing their music playing for not being quite his tempo, and Andrew is his biggest victim.

https://screenrant.com/whiplash-who-took-music-folder/