In response to the original question about controlling Nuendo (or any other audio software), I would recommend sending midi data and then using Spekenzie Labs serial to midi converter to translate the data and map it to an existing software midi port on your computer. I have successfully done this using HDD spindle motors.
The MIDI signals you will want to send are probably CC signals, here is the arduino code I used to send my signals.
Serial.print( 176, BYTE); //Control Change identifier
Serial.print( 70, BYTE); //Controller ID Number
Serial.print( 65, BYTE); //Controller direction number
I have two encoders, one is MIDI control 70, and one is MIDI control 71. These are rotary controllers in the MIDI standard, and most audio software will map to them. The direction number controls how far/fast values change. 64 is the middle, so 65 is really fast to the right or up, and 63 is really fast to the left or down. The range is 0 to 127, both of those being very very slow to the left/down (0) and up/right (127). It seems backwards, but it works!