The ornamental design for a case for headphones, as shown and described.
FIG. 1 is a front perspective view of a case for headphones in a closed position in accordance with the design;
FIG. 2 is a front perspective view thereof in an open position;
FIG. 3 is a front perspective view thereof in the open position, with environmental headphones;
FIG. 4 is a top plan view thereof in the open position;
FIG. 5 is a left side view thereof in the closed position, a right side view thereof in the closed position being a mirror image of the left side view of FIG. 5; and,
FIG. 6 is a left side view thereof in an opened position, with environmental headphones, the right side view being a mirror image of the left side view of FIG. 6.
The broken line headphones represent environment. The remaining broken lines illustrate portions of the case. None of the broken lines form part of the claimed design.