The ornamental design for a microphone, as shown and described.
FIG. 1 is a front perspective view of a mircrophone according to my design;
FIG. 2 is front elevational view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines in the drawings depict portions of the microphone that form no part of the claimed design. The uneven broken lines in FIGS. 1-4 depict boundary lines that form no part of the claimed design.