Krassyg,

In this context I don't think that there is any advantage in using a karaoke player. A true karaoke player would be able to throw song lyrics on to a screen. In your use case there will not likely be any screen. While in some cases a karaoke player could null some or most of the center vocal, the important part is to find karaoke source material.

Once you have the microphone and music signals, they can be combined with a simple mixer. This "mixer" could be a cheap commodity unit or a simple resistor network.

While the karaoke music source could be from a Sonos system, I don't recommend attempting to use a Sonos player to drive the speakers. The inherent 70ms delay between Line-In and output from the player will drive amateur singers crazy.