I claim the ornamental design for a music instrument mask, as shown and described.
FIG. 1 is a perspective view of the mask in use;
FIG. 2 is a front view;
FIG. 3 is a rear view;
FIG. 4 is a top view;
FIG. 5 is a bottom view;
FIG. 6 is a right-side view; and,
FIG. 7 is a left-side view.
The human form and musical instrument illustrated in broken lines in FIG. 1 represent environment during usage only and form no part of the claimed design. Features of the music instrument mask illustrated in broken lines represent no part of the claimed design.