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 view thereof;
FIG. 3 is a rear view of thereof;
FIG. 4 is a left side view of thereof;
FIG. 5 is a right side view of thereof;
FIG. 6 is a top plan view of thereof; and,
FIG. 7 is a bottom plan view of thereof, the portions shown in broken lines being for illustrative purposes only and forming no part of the claimed design.