We claim the ornamental design for a speaker system for portable device, as shown and described.
FIG. 1 is a perspective view of a speaker system for portable device of the present invention;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a first side view thereof;
FIG. 5 is a second side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line showing of speaker system for portable device is included for the purpose of illustrating environment and forms no part of the claimed design.