The ornamental design for a speaker, as shown and described.
FIG. 1 is a front perspective view of a speaker showing our new 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 view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a cross-sectional view of the dot-dot-dash encircled portion in FIG. 2.
The dot-dot-dash broken lines encircling a portion of the claimed design shown in FIG. 2 that is illustrated in a cross-sectional view form no part of the claimed design. The short dash-dash broken line shown in FIG. 8 represents environmental structure only and forms no part of the claimed design.