I know this will sound a bit crazy, but connect the AC3 output to the audio input of a VCR or tape deck with VU meters if you've got one. (Keep your amp shut off or tv speakers turned all the way down to be on the safe side.) If anything is coming out at all, the meter ought to do something.

I've heard the AC3 signal by accident a couple times with my old analog only player before switching to the mono track. Sounds a lot like a fax machine or dial up modem!

What model demodulator did you finally end up getting?