FIG. 1 is a front and right side perspective view of a whistle mouth showing my new design; FIG. 2 is a right side elevation view thereof, the left side being a mirror image thereof; FIG. 3 is a top plan view thereof; FIG. 4 is a front elevation thereof; FIG. 5 is a bottom plan view thereof; and, FIG. 6 is a back elevation view thereof. The portions of the drawings shown in broken lines indicate surrounding structure which does not form part of the design.