I googled around and could not find anything good.

Even just a schematic would help.

All I want is a dinky thing to clip on my phone, transmitting I don't care wirelessly over to a dinky thing with a speaker (or wire into into the S1 Speaker) so I can send my voice over there.

One way is good enough, but I'll consider a two-way.

I thought about stripping down a baby monitor but would prefer a neater solution.

Anybody got any tips for this?

FWIIW (and unfortunately not an answer to your question), the S1 already has everything in place to be able to do that. (i.e. it has a microphone and a speaker). What is missing is software support. This would be a good thing for DJI to enable.

Yeah I know thus I am considering a little electronic gadget to work around this (current) limitation

Hello and good day MarkusXL. Thank you for reaching out. I hope that you will get the best recommendation with regards to this matter. Thank you for your support.

Wah.  It turns out the simplest cheapest way to throw your voice is AM radio.  There are circuits out there, but I decide to Pass.

I'll wait for DJI to unlock the Speaker for us.

Doo dee doo dee doo dee doo...