The ornamental design for a decorative sound spectrum display, as shown and described.
FIG. 1 is a front elevational view of decorative sound spectrum display showing my new design, the cord is drawn broken away for purposes of illustration;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a side view, taken from the left side of FIG. 1, the right side view being a mirror image thereof;