The ornamental design for a diaphragm for speaker, as shown and described.
FIG. 1 is a perspective view of a diaphragm for speaker of my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a perspective view showing the diaphragm for speaker being mounted to a speaker frame which is shown in broken lines for illustrative purposes only and forms no part of the claimed design.