@Julien-0 well i just tried it myself for the first time.
Headphones + cable +jack to usb-c into FP4
and it didnt work. Dont know if it 's a problem from your/our instalation or something that hasnt been fixed yet.
I found a post on the forum https://forums.ubports.com/post/77107 with a suggestion on how to manually adjust it. But not sure if that works in your/our case, or if one should play around with things that might affect your daily usage. Bluetooth connection to headphones seams fine, well sometimes I need to reconnect them to have all buttons recognized
but that a small issue.