The ornamental design for a microphone, as shown and described.
FIG. 1 is a perspective view of a microphone showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a side elevational view of one side thereof;
FIG. 5 is a side elevational view of the opposite side thereof; and,
FIG. 6 is a top plan view thereof, the undisclosed bottom is not part of the claimed design.
The broken line showings of the circle and the switch on the front and the small circle and X on the rear are for illustrative purposes only and form no part of the claimed design.