The ornamental design for a speaker, as shown and described.
FIG. 1 is a front perspective view of a speaker;
FIG. 2 is a front view of the speaker of FIG. 1;
FIG. 3 is a left-side view of the speaker of FIG. 1;
FIG. 4 is a right-side view of the speaker of FIG. 1; and,
FIG. 5 is an enlarged front perspective view of the speaker of FIG. 1.
The dash-dot-dot-dash broken line immediately adjacent the solid-line portions of the design represents an unclaimed boundary of the design. Features shown in other broken lines in FIGS. 1, 2, 3, and 4, form no part of the claimed design.