Using 5.8ghz for video is, surprisingly, easier than using it for audio in a number of ways. Most FPV pilots don't really care much about the audio coming through on their video feed, so most VTXs don't really focus on that. Indeed, most these days only use their audio channel for "SmartAudio" commands from the aircraft's flight controller, and many VTXs (like mine) don't transmit any actual audio coming in via the SmartAudio pad, using it only for digital data.

My VTX did have a built-in microphone, though, so I desoldered that and used a voltage divider and coupling capacitor to take the headphone signal coming out of the Rpi and drop it down to mic level signal.

...which turned out to be pretty unnecesary, somehow. Messing around on my breadboard, I found that with a proper voltage divider the audio was far, far too faint to overcome the noise on the audio channel. Just pumping line level into the mic pad on my VTX worked fine. Even the coupling capacitor didn't seem necessary, though it also didn't seem to make things worse, so I left it in. The resistors are just to help crush the stereo down to the single channel I have at my disposal.

Fig. 1: messing with the audio circuit on the breadboard

Anyway, I now have the option of getting audio and video through the wireless signal, plugging headphones into my goggles.

Another challenge I've run into with this compact build (and even my larger one) is the sheer massive size of commercially available switches. Sure, they're not big for like...a car, and you can get truly tiny SMD switches, but they're never designed to be on the exterior of a device and usually lack any detent or a toggle you can actually grip without the help of a switch part designed into the larger device.

After searching for hours and hours on every site I could think of, I gave up on finding a toggle switch that was low profile and would do what I needed...then saw a USB hub on a website I was shopping that had 7 of them.

These USB hubs are cheap and ubiquitous. Presumably they just know where to find these switches; I don't, other than "cheap USB hubs".

Fig. 2: The smallest rocker switch I could find anywhere, next to the ones built into this no-name USB hub.

Fig. 3: I don't know where you came from, tiny switch, but I love you.