The ornamental design for a speaker, as shown and described.
FIG. 1 is a front perspective view of a speaker, such as a Bluetooth speaker, which is capable of playing sound, in accordance with our new and ornamental 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; and,
FIG. 7 is a bottom plan view thereof.
The broken lines depict portions of a speaker in which the design is embodied and forms no part of the claimed design.