I've made an audio meter with my Arduino and a sexy OLED display. The audio input is a simple circuit I found on Google:
It works really well. My question is: if I connect this circuit to the wires between my computer and my powered monitors using a simple Y-splitter cable, will my circuit affect the audio going to the speakers? I'm not worried about a little attenuation, but is there a chance this circuit will interfere with the character of the audio - the frequency response etc?
It will affect the frequency response if you have the potentiometer turned close to either extreme. In that case, you’ll have a 10uF capacitor with a small series resistance to ground or Vcc, either of which will likely affect the high frequencies.
If you merely put a 10k resistor in series with the 10uF capacitor, it should completely remove this effect.
Replace the potentiometer with a resistor divider made of two 47k resistors?
You'll get 2.5V offset and no danger of loading the audio signal. The 10uF capacitor
can be a lot lower, down to 0.47uF or so without affecting the audio frequencies.