Audio/Video Mixing

Any body has experiences on how to add audio/video mixing feature to the
Licode MCU?

I see VideoMixer in erizo source code but it is more like an skeleton
without functionality. I know mixing/transcoding is not a priority in
Licode project but it would be very useful to have a sample code, even a
simple one like audio mixing, as a start point so developers be able to
extend it or implement their own.