We claim the ornamental design for a speaker, as shown and described.
FIG. 1 is a front perspective view of a portable speaker;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a right side view thereof;
FIG. 8 is a bottom perspective view thereof; and,
FIG. 9 is a rear perspective view thereof.
The broken lines shown are included for the purpose of illustrating portions of the speaker that form no part of the claim. The short dash-long dash lines are boundary lines which do not form part of the claimed design.