IMHO, avoid integrating karaoke to AV at all cost.

Down here in Houston, I've been called to see total disaster caused by those add on Karaoke pieces.  Consumer end AV gears are not designed to handle those microphone input.  I do not know how to explain this.  But I saw speaker tweeter dirvers got busted, AVR got clipped....

Best way to deal with is total separate Karaoke system with its own speakers, but you can share the display.

Link to place that you can see different gears/options on karaoke needs. [Link: phonglekaraoke.com]