@ancientsounds It involves stripping the phone out to the motherboard (no need to remove the battery if that is OK), removing the motherboard and cleaning any dust out. On some I have added a thermal pad (like for PC CPU's) for the Snapdragon CPU - this is usually a last resort.
@Fuseteam tested on my FP4.
The switch beetween headphones and speaker is automatic, music playing works well but when I have a call it switches back to phone speaker.
I have to re-enter this commandline after reboot : pacmd load-module module-udev-detect
@arubislander Thank you. I was trying that several times, and getting rather anxious about it not rebooting, but then finally it did reboot and all is now well again.
But I shan't be trying to use the Wireless Display again very soon ...