According to this article from Post Magazine, there were no instruments, but there were guitar, bass, and drum vocalizations, plus a beatbox. With the vocalization technique, apparently you are singing through an instrument, so it is still your voice rather than the instrument that is causing the sound (think Peter Frampton and his talk box).
Kevin O’Connell, re-recording mixer at Universal Sound Studios,
handled the music and dialog mix on the film. For each of the 80 songs
delivered for the final mix, there were up to 120 tracks of a cappella
singing and vocalizations. O’Connell said, “I would have the main
characters on the first 10 to 12 tracks, and the secondary characters
on the next 10 to 12 tracks. Then I’d have scads of background vocals,
beatbox, guitar vocalizations, bass vocalizations, and drum
O’Connell took the principle tracks and panned them around the stage
to match where the characters were dancing while they were singing.
Next he filled in the background characters vocals. Then he filled
the track in with the different instrument vocalizations of guitars,
drums, bass, beatbox, and any other vocalized samples that came with
the song. He said, “When you see it, you’ll find it hard to believe
that there were actually no instruments during any of those songs.”
The article tells all about the recording and mixing processes for the film - very interesting!