I claim the ornamental design for a portable karaoke system, as shown and described.
FIG. 1 is a front elevation view of the portable karaoke system showing my novel design;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is a left side elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a top elevation view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a perspective view thereof; and,
FIG. 8 is a perspective view thereof.
The broken lines depicting surface details in FIGS. 2, 5 and 6, part of two microphones shown in FIGS. 7 and 8 and a music player in FIG. 8 are for illustrative purposes only and form no part of the claimed design.