Hi AlexFr. I also got the USB-C to mini jack adapter, and I have the same two issues as you.
I’m using the USB-C to mini-jack adapter supplied by Fairphone, shipped on 25 October 2023. Some observations below.
Any help from the Fairphone experts would be much appreciated!
Problem 1: Very low headset microphone level.
The headset is a set of Sony wired earbuds with mic and 3.5mm TRRS plug. On phone calls, I can hear the other person but they cannot hear me (or only very faintly). The headset does not have this problem when used with other phones or laptops. The other person hears me fine.
Problem: Phone not sending sound to external speakers
I have two external speaker systems, one in my car and one at home. This problem occurs with the car system, but not the home system. Both speaker systems use a 3.5mm TRS plug.
When I first plug in the USB-C adapter (with no speakers connected), the FP5 shows the notification “Charging connected device via USB.”
Then if plug in the working (home) speakers, this notification disappears and the audio is routed through the external speakers.
But if I plug in the problematic (car) speakers, the notification remains, and the audio output is through the built-in FP5 speakers.
The car speakers work fine with my other devices.