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;
FIG. 5 is a right-side view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof;
FIG. 8 is another perspective view thereof, in which a display device in broken line is provided to illustrate an example environment;
FIG. 9 is an enlarged view of the encircled portion A in FIG. 1;
FIG. 10 is an enlarged view of the encircled portion B in FIG. 2;
FIG. 11 is an enlarged view of the encircled portion C in FIG. 3; and,
FIG. 12 is an enlarged view of the encircled portion D in FIG. 5.
The broken lines are for environmental purpose only and form no part of the claimed design. The shade lines in the figures show contour and not surface ornamentation.