Upon the advice of this question/answer I have added a 1:1 audio isolation transformer to my circuit.
I picked up a transformer at the local radio service shop and dropped it into my bread-board prototype; it's a Shaffener IT237 as shown in the spec sheet below. It works well in my application. I'd like to purchase an SMD equivalent, but I'm not sure what paramaters are most relevant for this circuit.
I don't fully understand how this transformer affects the circuit, but I do know that it fixes the issue outlined in the post linked above.
The circuit shown below is a simplified version of my project. The input is the line-level AC coming from a CD player, ChromeCast, or similar appliance. The output of the Class A amplifiers is sampled by an Arduino.
What parameters of the existing transformer shown in the specs sheet below should I consider when looking for an alternative SMD version of this component? I would guess that I need a low impedance device as this is for use with very low voltages (5-50mV), but beyond that I have no idea what I'm looking for.