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;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a reference view thereof showing that the upper part has been open; and,
FIG. 8 is a reference view thereof showing an example of the usage of the subject design in which a bluetooth headset is attached to the subject design.
The broken lines depict portions of the speaker that form no part of the claimed design.