The ornamental design for a portion of a loudspeaker, as shown and described.
FIG. 1 is a perspective view of a portion of a loudspeaker showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a side elevational view of one side thereof; and,
FIG. 6 is a side elevational view of the other side thereof.
The broken line showings of the plaque on the top and the remainder of the loudspeaker are for illustrative purposes only and form no part of the claimed design.