The ornamental design for a soundamplifier, as shown and described.
FIG. 1 is a perspective view of a sound amplifier showing my new design,
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side thereof;
FIG. 6 is a front view thereof; and,
FIG. 7 is a rear elevational view thereof.
The broken lines shown in the figures depict portions of the sound amplifier and form no part of the claimed design. The dot-dash lines depict a boundary of the claimed design and form no part of the claimed design.