He was telling everyone to play middle-C for tuning purposes. (Source: Miles Teller tweet)
More than likely Fletcher took it because he had said he did not want to see another folder left laying around. Another possibility is that Andrew either hid the folder or lost it on purpose to sabotage Tanner and take his place as the core drummer.
In the screenplay, it's revealed that a janitor threw out the folder on accident. It's likely in post-production this bit was cut to make it seem more malicious.
Andrew, Fletcher, Jim Neimann, Nicole, Ryan, Carl Tanner, Uncle Frank, Aunt Emma, Dustin, Travis, and others
What is the plot of Whiplash?
A promising young drummer enrolls at a cut-throat music conservatory where his dreams of greatness are mentored by an instructor who will stop at nothing to realize a student's potential.
What was the budget for Whiplash?
$3.3 million
How much did Whiplash earn at the worldwide box office?
