Thanks Linuxmusician01, but sadly I don't think either of those devices will work for me. The first USB stick won't work since I need stereo input, yet the microphone input is mono, not stereo.
The second one would be ideal, but it is audio output only, there isn't even a mono microphone input.
Let me know if you have any other thoughts.
Since I posted yesterday, I did find this device on Amazon
It looks like a lot of soldering will be required, not the least of which is to solder on a GPIO header. and maybe even RCA female connectors:
It does claim to have a stereo input though: "...Stereo input and output, including a microhpone input (microphone supplied)..."
Besides the soldering needed to make this work, it looks like the documentation is atrocious. In the Amazon comments, I see that some people have gotten it to work and confirm it works on the Raspberry Pi Zero W, and yet there are others, including a long-time embedded engineer haven't:
[quote=Jordan]No support for this besides the forum, which is like listening to crickets chirp
March 28, 2019
It looks like a really nice board, and the price is right. I especially like the off board RCA jacks. However I while I am a Raspberry pi newbie, I do have some experience with software. I am also a professional embedded hardware engineer (26 years now), but still I was not able to make this work.
The software says it's set up, but it will not get recognized by the Pi. Flatmax did try to help me once to no avail. The setup guide is not easy to follow or understand, and the software from the website is not good.
Maybe someday the setup guide will not say things like "Download and install the attached deb file" with no other explanation. Maybe someday I'll be able to get this thing working. I wish I could rate this higher but as it is, it's just not ready for most people.[/quote]
So, I'm still looking. I don't need an internal solution like this audio injector above. That USB stick form factor is perfectly fine with me. But I definitely need a stereo input. A single mono microphone input won't do.
Let me know if you have any other ideas! Thanks!