Hmm. . after finding out more info I think that the port has video and audio inputs not outputs.. I'm guessing that a 5200 Expansion module would have gone here. Still looking, not much info about this out there.

from the 7800 faq at Atariage

model revisions:

Atari CX7800 - Vastly modified and enhanced 2600 model. Two joystick ports on lower front panel. New chipset (except sound chip; pokey chips could be added to carts). Side expansion port for upgrades and computer add ons. Bundled accessories included two CX24 ProLine joysticks AC Adapter, switchbox, RCA connecting cable, and Pole Position II cartridge.

Atari CX7800 - Second Revision. Slightly revised mother board, added an addional timing circuit. Expansion port connector removed from mother board but is still etched. Shell has indentation of where expansion port was to be.

Atari CX7800 - Third Revision. As above but only a small "blemish" mark on shell.
Also this info..

What 7800 hardware was announced, but never released?
7800 Keyboard, peripherals and computer games designed by GCC. There was also a 7800 adapter for the 5200.
Audio info:

Expansion Port Audio
There is no audio signal output on the expansion connector pin 17. Introducing an analog audio signal on this pin produces no results, whether this is because there needs to be a flag set in one of the I/O chips to enable external audio mixing (which I suspect is the case) or whether it is expecting a digital signal, I don't know.

In any case, you can't pick up an audio signal from here for a separate audio output.