I claim the ornamental design for a sound generator, as shown and described.
FIG. 1 is a front/right/top perspective view of a sound generator (a front/left/top perspective view would be a mirror image thereof);
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The stippling in FIGS. 1-7 represents a contrast of color.
The broken lines representing the feet, wiring, and other structures are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.