In response to DVDdoug:
The audio part of the mixer is already set.
Do you already have the mixer with the motorized faders? Or, someone else is working on that part of the design?
Somebody else did it already. Audio from mixer is working. I just need to use rest of the fader pins to motorize it.
Of course, you'll need to check the number of inputs/outputs and I'd guess you'll need more than one Arduino. In theiry, you could multiplex (basically time-share) the ADC inputs for fader position since these don't change at "microprocessor speeds", but that may not be worthwhile.
I think I could use 3 arduinos, one each 8 faders. I would avoid multiplexers but still a good option.
One thing to watch-out for is electrical noise. Audio circuits are very sensitive because they handle a wide signal range from a few millivolts to a few volts. If you get a few millivolts of "digital noise" leaking into the audio, you'll hear it (when the volume or signal level is too low to drown-out the noise). So if you use PWM on the servo motors, you'll probably have to filter it. You might even need to filter DC to the servos so you don't get a "click" when the motor current turns on & off. And, use a separate power supply (at least separate regulators) for the digital stuff, use shielded cables, and just try to isolate and sheild the digital from the audio as much as possible.
Thanks a lot for this advise! I will make a test as soon as I can to see if some noise gets in the audio track. Do you think that it could increase depending of the total amount of faders connected to the same board? I mean, can I test the circuit with just 1 fader or I should connect more to have a final result?