The first thing you'll need to do is sort out signal level translation, the Arduino only works with 5v, outputting 12v or 7v through it will kill it (Shift_Down)
I guess the best thing to do would be to feed the output from the selector (I'm not sure why it's labelled as Audio?) through a voltage divider to divide it by 3, giving the maximum voltage as 4v. Analog pins can cope with the variation below this fine.