The ornamental design for a speaker, as shown and described.
FIG. 1 is a front view of a speaker.
FIG. 2 is a back view thereof.
FIG. 3 is a left side view thereof.
FIG. 4 is a right side view thereof.
FIG. 5 is a top view thereof.
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a perspective view thereof.
The broken line showing in FIG. 2 of the interior structure and patterning within the four rectangular elements is included for the purpose of illustrating portions of the speaker and forms no part of the claimed design.