The ornamental design for the rock speaker, as shown and described.
FIG. 1 is a front perspective view of a rock speaker showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines showing of battery box, solar panel, and the speaker are for illustration and environmental purpose and form no part of the claimed design.