The ornamental design for a speaker, as shown and described.
FIG. 1 is a front perspective view of the speaker;
FIG. 2 is a left side view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line showing of the rear and bottom of the speaker is included for the purpose of illustrating environment and forms no part of the claimed design.