Using Mac as an Amplifier and Karaoke [PLEASE HELP!]

Hi Guys,

I used to be able to plug my headphones and microphone into my PC and was able to listen to myself while speaking on the microphone. If I played a karaoke/instrumental track I was able to listen to myself and the track at the same time both using my headphones or without them. In other words, my mic was a input device which would also come out live through the speakers.

(Sometimes I would get feedback but that was easily controllable)

I have tried everything and I cant find the way to make my mic input to come out live as output at the same time. I guess is some sort of firewall or feedback protective feature?

I bought different types of mics (USBs, Macs, etc) but nothing seem to work. All the mic work because I can record on GarageBand, Use Skype, etc. It's a matter of allowing the system to let the input go through to output at the same time so that I can sign karaoke.

Please help!