these are nowhere near able to do this kind of audio processing.
I have done similar project on Arduino Uno (with less channels & quality) and based on that experience 100MHz DSP should easily be able to do this,
I can code the DSP with C/C++ or assembly for performance.
All the mixed channels are digital and the sample data for the channels should reside in external memory because there can be several MB's of data.
With a 16MHz processor??? Even the Due only runs at 84MHz.
That rules out the Arduino.