I've got an Arduino Uno with the Adafruit Wave Shield. Taking inspiration from one of Adafruit's demonstration projects, when I constructed the shield I wired from a resistor on the audio shield to A0 so I can measure the amplitude of the sound being produced at any time. The specific text from that article is:
The plan is to make the pumpkin's mouth animate with the sound. Although we could do this all in 'software' it turns out to be a heck of a lot easier to just use an analog input to measure the output of the amplifier. So connect a wire from the 1.5K resistor back to analog input #1
So I did that, and it works fine.
In my current project I've run out of pins on the Arduino when using the shield so was thinking of adding an ATTiny85 to give me 3x PWM outputs to run some additional RGB LED effects based on the same analog input. Will I need to take any additional precautions so that A0 on the ATmega328P and A3 on the ATTiny85 don't interfere with each other - or is there nothing to do but join them both to the same point on the audio sheild?