I have now tried the original Fairphone USB-C to Mini Audio Jack adapter. The sound quality is good for me personally. 
However, I certainly don’t have a “fine” ear for music. Therefore, I cannot judge whether it is also good for someone who has very high demands on music quality and hears every fine nuance. For me, at least, it’s enough to listen to music on the side in everyday life.
I have already written something here about the controls: