I claim the ornamental design for a musical instrument covering, as shown and described.
FIG. 1 is a first front view of a musical instrument covering showing my new design in use;
FIG. 2 is a side view thereof;
FIG. 3 is a plan view showing the musical instrument covering flat; and,
FIG. 4 is another front view of FIG. 1; the broken lines showing a guitar and a stand representing environmental structure and forming no part of the claimed design.