The music box at sea dogs rest seems to be just there without any reason at all at first. However, there is more to it than meets the eye.
Everytime we finish an objective, the music box changes to help us.
The several shanties it played helped us:
We shall sail together: Retweet a post... huh. Seems familiar to an another moto We shall tweet together
Seek the dead: Look for graves
Row row yer boat: Look for crashed rowboats
Ballad Of The Mer: Look for notes in the siren shrines
Happy Birthday: ???
I think DeMarco was on a mission from his father (Ramsey was the reason DeMarco closed arena if you did not know) but he disobeyed him and his soul somehow ended up inside that music box. Now he is trying to help us by playing fitting shanties.