MattiasOfTheMetal:
If I'm reading right on the datasheet you attached for the CD405(x) series, the max voltage to pass through a pin is 15vdc, but I don't see anything about ac. And it looks like the min voltage it can handle is 0v, to the voltage supplied to it. I'd have to run it directly off 9v, and command it with 5v from the arduino, for it to handle the 9v dc power, but what would I do about the ac signal?
You need to think about biasing (i.e. giving it a DC V level to sit on top of) the AC signal if you want to pass the full AC signal (positive and negative cycle) through the 4051 working on a +9V single supply rail.