The ornamental design for a speaker, as shown and described.
FIG. 1 is a top, front and left side perspective view of a speaker showing our new design;
FIG. 2 is a top, rear and right side perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is an enlarged view of portion 9 shown in FIG. 1.
The broken lines depict portions of the speaker in which the design is embodied that form no part of the claimed design. Additionally, the dot-dash broken lines encircling an enlargement portion of the design forms no part of the claimed design.