Progress in that it's been started, and that it looks like it might work... but it performs pretty unsatisfactorily so far. ;) Very little recovered audio.
At least I think I know why, my (semi-informed) hunch is that the bandpass filter I used, (this one) instead of the one Pete specified, likely has too much insertion loss. I have the instruments to prove this out, just haven't bothered yet.
This has got to be the problem, I've ruled just about everything else out!
At first I thought, could I have messed up in my construction of the mixer? I swapped the homebrew mixer module for another with an SBL-1, nope, exact same poor performance.
Then, I considered the possibility of insufficient LO drive. So I added a N3ZI-style buffer amplifier between the DDS and the mixer, as 250mV p/p going into the mixer straight from the DDS may have been a bit anaemic. Nope, didn't help either.
Also tried this RF amplifier between the BPF and the mixer. This helped a little... when it didn't take off and oscillate, which wasn't very often.;) Now I know why there was supposed to be an attenuator in that amplifier circuit, need to go back and put that in!
Its definitely not the audio module, that thing is HOT!
I need to quit procrastinating and fire off an order to Mouser and obtain the proper IF cans to make the specified BPF. I also need to put the thing on a bigger breadboard, build a voltage regulator/power distribution module, clean up the Arduino/DDS section and switch to one of my new Pro-mini-328's instead of the big old clunky Diavolino.
I may yet work a deal with Santa in that Adafruit sends me a couple of those really nifty tiny OLED displays!