Update: You can use AirParrot to get the desired effect, but the comments below about your microphone still stand.
AppleTV streaming can have up to a 2 second delay. Imagine singing into your microphone and only hearing your voice two seconds later, after the lyrics are long gone, and no longer in sync with the song?
While I think it is technically possible to enable AppleTV streaming, it would not make sense unless your microphones were plugged into the same audio mixer as your TV, which is just about never the case.
Almost always, what you *really want* is to have the computer's audio goes out to the mixer, the mics go to the mixer, and the mixer goes out to the speakers.
Unfortunately even in that case, with the video on a 2 second delay, the audio will be *ahead* of the video.
Currently, there is no feature for this in kJams, cuz everyone i've asked says "no, we do NOT hook the mics to the same amp that the TV is connected to". usually because the TV has built in speakers.
If you want to try it anyway, it is possible to use iTunes to send the song to AppleTV, but you'll still have a problem getting the audio to mix in correctly, that's up to you. So to try it out, export your karaoke songs to QuickTime, then load the QuickTime files into iTunes and play them thru AppleTV from there. To learn how, see the article on Exporting.