Discovered that MMC5 audio output is Pin 3. The only game I have tested is "Just Breed". In case your wondering how I tested... I modified an Uncharted Waters Cart (ETROM), to use EPROMs (I can post info on this, if I get requests), glued an extra pin to the cart edge connector. I used pin 18 (EXP 2), because nothing that I know of uses it. Then soldered pin 18 (on the inside of the NES) to Audio Out with a 1K resistor.
Since my NES now has an Audio in (like a Famicom), I modified my Famicom -> NES converter (FAM Ping 46 -> NES Pin 18) so I now how sound from special carts like Akumajo Densetsu.
I can post pics and docs, if you like.