The sound board is called the "Audio Companion" board. It has 4 input pads with three different gain levels. The inputs are high impedance and should not affect normal operation of the sound circuits, so you don't have to cut and disable existing sound circuitry. The different inputs allow you get proper mixing of sound sources and they were chosen specifically for direct Pokey/TIA output (Inputs A and B), the 7800's cartridge audio line (Input C), and the 400's SIO Audio circuit (Input D). There is also a GAIN pad which, if cut, allows you to boost the output.
You can use extra inputs for additional sources if you wish (I'm experimenting with adding the keyboard click on the 400).
The board is primarily designed for machines that shipped with only RF output: 2600, 5200, 7800 & 400 (the domestic 600XL didn't have audio, but the signal is still there on the board). It can be used in other machines as well as it's basically a 5V powered buffered audio mixer.
The boards are installed by soldering them across a 5V decoupling/noise filter capacitor. The board's polarity is marked and you must pay attention to orientation to avoid damage. The input and output taps are then available at the top.
The boards are $10 each and are available now. Unfortunately, UAV's aren't shipping again until November, so you can get an Audio Companion shipped now or with your UAV. Thanks!