(I probably shouldn't reply since it's been almost a year since I used MJ8--I use MC9 now--but being no one else has responded, here goes...)I don't think so. The only thing even remotely related would be to record stuff coming in from Line-In, but there's no way to echo it out via MJ8.
In other words, there is no "karaoke" function available since that's what you are really looking for.
Now, I don't know about your laptop and its audio setup, but on a PC with soundblaster, you can tell the system (via either window's mixer or the soundcard's mixer) to echo out the microphone input. Usually it's just a matter of "unmuting" it.
One big problem with this solution, though, is feedback.
But nothing ventured nothing gained, right?
Omni