This project demonstrates how the xr package can be used on Overleaf to resolve labels in other .tex files that have not been \include-d in the main file.
Make sure you enable the correct system call in latexmkrc, depending on the LaTeX engine you're using!
LianTze Lim (from an answer by cyberSingularity at http://tex.stackexchange.com/a/69832/226)
This example demonstrates how to write polyphonic or instrumental music on Overleaf, using M-Tx (.mtx) syntax.
This was made possible with a custom latexmkrc file. The music piece examples here were taken from the M-Tx documentation.