The ornamental design for a speaker, as shown and described.
FIG. 1 is a perspective view of a speaker showing 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; and,
FIG. 7 is a bottom plan view thereof.
The broken lines adjacent to the shade lines form the boundary of the claimed design. All other broken lines are shown for illustrated purpose only. None of the broken lines form part of the claimed design.