Bendy and the Ink Musical is a soundtrack and an Easter Egg from Chapter 3 of Bendy and the Ink Machine. It is an edited version of the song from the fan-made music video uploaded by the Youtuber Random Encounters.
The music itself actually turns out to be an old fashion-styled instrumental and shorter version of the noncanonical song "Bendy and the Ink Musical".
There is a music radio on a box next to some instruments in Level 11 from the last room of the second stairway sets. Clicking on it plays the track.