I haven't purchased my A139 yet, but was looking through my box of cables to see which one might work for the AV output as I am leaning heavily towards it. Based on what you described with the video showing up in the right audio position of your cable it probably is using what some call an old Sony Camcorder cable 3.5mm layout (see attachment). Example of cable at Amazon. I am curious as to what audio out would be needed for as I assume the video playing is just whatever the "Live Video Setting" is currently configured and hearing the interior microphone in slight echo chamber mode would be useless/annoying, which would probably explain not getting signal on other RCA outputs. It might even be the case that we could use a mono 3.5mm plug to RCA like this one so that it would be a smaller cable to route. I will try these options out once I order my A139, but it will likely be a few weeks as bunch of other projects.
It is only vide output without sound
you need OMTP pin type
The video resolution is low
If your cable doesn't work . You can cut and reconnect the wires to get Compatible RCA AV cable ( Switch the wires between 1 and 2 )