The ornamental design for a speaker, as shown and described.
FIG. 1 is a front perspective view of a speaker showing our new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a front elevation view thereof;
FIG. 6 is a rear elevation view thereof;
FIG. 7 is a left side elevation view thereof; and,
FIG. 8 is a right side elevation view thereof.
The broken lines shown to be immediately adjacent to the shaded/contoured areas represent certain portions of the bounds of the claimed design while all other broken lines are directed to environmental features and are provided for illustrative purposes only; the broken lines form no part of the claimed design.