The ornamental design for the frame for mallet instruments, as shown and described.
FIG. 1 is a top front right perspective view of the frame for mallet instruments;
FIG. 2 is a left side elevation view of the frame for mallet instruments shown in FIG. 1, the right side elevation being a mirror image thereof;
FIG. 3 is a top plan view of the frame for mallet instruments shown in FIG. 1;
FIG. 4 is a front elevation view of the frame for mallet instruments shown in FIG. 1, the rear elevation view being a mirror image thereof; and,
FIG. 5 is an enlarged view of the upper portion of the left side elevation view of the frame for mallet instruments designated by circular broken line in FIG. 2, the upper portion of the right side elevation view being a mirror image thereof.