This seems to be a not uncommon problem that has been raised on the forum previously but there is no conclusion found for it.
Some time ago audio stopped being played through my wired headphones.
I have two known good USB-C to 3.5mm adaptors, both are charge and listen adaptors, and several known good heaphones, plugging them into the Fairphone 4 results in audio continuing to play through the phone’s speakers. The device does still charge through them. One of these previously worked fine, for ages.
This is without messing with any of the developer options, and running the stock OS. Someone suggested the issue may be related to Android 13 but some reports of the issue well pre-date that.
Any ideas? It’s a function that I need to work.