Hi,
I used a standard sound card but then plugged that into a Karaoke mixer, using a Radioshack cable with RCA plugs on one end. Mic inputs then go to the mixer not the pc and out to a amp. However you could route it back to the pc via line in if you wanted to use your pc speakers.
As for putting it in your Mame cab that is a hard one my mixer for one would not fit in a mame machine. I do have it in a cab but it is bigger than a mame machine. Bottom line if you want a karaoke machine build a new cab! I did.......
You can buy sound cards with 1/4 inch jack inputs on them, I had one but ended up not using it.
Gary