It looks to me like the RCA cables terminate in a 2-pin plug at the mystery board, so my guess is they are wired together and bring the (mono) sound out to a stereo amplifier. The 9-pin plug is probably video for a TTL RGB monitor / projector, so neither NTSC or PAL.
Wow, I think you are absolutely right. So here's the question ... why such a huge board for a simple TTL adapter? Pina shows how to construct one in his book that consists of a single chip on a tiny board. What exactly is this board doing then? That giant capacitor alone has me stumped.
Here's a question, what if that isn't audio at all? What if it is a composite video output, that would certainly explain the additional chips if it was converting to NTSC or PAL. Albeit I am stumped as to why it would be split unless it was used to drive more than one monitor. I can't tell from the picture how far up the cable the split occurs. But seriously why would a board of this nature need to provide a separate sound output? The Mac already had a decent sound output. Just pass the sound through this board and take it off the jack – one less wire to run out of the case. Unless the board modified the sound in some respect, but I'm not sure how as there's no way to control it. I suppose it could drive sound off the Mac without bypassing the Mac's built-in speaker which plugging into the audio jack would do.
Let's flip it around ... what kind of inputs could a card like that provide for the Mac? It's situated between the logic board and analogue board, so at best it could allow the Mac to act as a monitor and speaker, but it couldn't send anything to the logicboard, so no data. But who would want such a thing? Perhaps a way to switch between an Apple II or a PC using the Mac as the display? But who would want a 9" B&W monitor when they could go the other way and have a larger color monitor?
Osgeld has a point too, that there could be an NTSC or PAL signal coming off the 9 pin connector like the Apple IIc & IIGS, but the Apple IIc/IIGS used a standard 15-pin connector, so if that's the case here, it would be some unique custom adapter, long since gone, and not compatible with anything Apple sold. Did the PC world use anything like this as a video output standard interface?
I also agree with Osgeld, it looks like a startup company, limited run kind of thing, not quite as crude as some prototypes I have seen. The 11-pin connector is a good indicator. I wouldn't expect a prototype to use such a finished power connector cable.
I have another question which is whether NTSC or PAL would use more or less circuitry, or about the same?