It's better not to use ht speakers for karaoke cause ht speakers do not
contain overload protection circuit. You may blow tweeters since most of the
energy of human voice concentrate at high frequency. Some people do share
speakers and they may get lucky but I have seen plenty of such examples.
Your luck also depends on your singing style:-).
If you really want to, you can add a compressor/limiter before the output of
mic enter amplifier.