The ornamental design for a speaker, as shown and described.
FIG. 1 is a perspective view of a speaker showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof where the right side view is a mirror image; and,
FIG. 5 is a top plan view thereof where the bottom plan view is a mirror image.
The dashed lines shown in the center of FIGS. 1-5 represent a symbolic break in the length of the speaker. The appearance of any portion of the article between the break lines forms no part of the claimed design.