I am trying to connect my laptop(from headphone jack) and my karaoke machine(audio output) into a single audio input. I was told that the karaoke machine and my laptop have different impedances, so the headphone jack would need to be converted into a different type of signal. Does anyone know what part I am supposed to use?

Thank you,

This classic article might be a good place to start a search for a passive summing box. You can build one of these things very easily from stuff you can get at an electronics supplier. There are several proper active solutions which will deal with the impedence difference 'properly', but a passive solution will almost certainly be good enough for what you need.