The ornamental design for a displayscreen with a graphical user interface for controlling sound, as shown and described.
FIG. 1 is a front view showing a first embodiment of a display screen with a graphical user interface for controlling sound; and,
FIG. 2 is a front view of a second embodiment thereof.
The peripheral dash-dot line showing of a display screen around FIGS. 1 and 2 is included for the purpose of showing a portion of the article which forms no part of the claimed design. Dotted lines are intended to show unclaimed portions of the design.