The ornamental design for the speaker box for an automobile, as shown and described.
FIG. 1 is a perspective view of a speaker box for an automobile showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a right side view thereof; and,
FIG. 6 is a left side view thereof.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are included for the purpose of illustrating portions of the speaker box for an automobile; the broken lines form no part of the claimed design.