Everything looks like it is in place.
Earphones powered up
Computer Bluetooth turned on
Connected to the only Bluetooth device that I have linked
Try sound (eg. YouTube video) > Sound from external speakers only
Ubuntu > Settings > Sound > Output Device > "Headset - (E8A) +61 428 279 021" (the linked earphones) > Test > The sound only comes out of my external speakers
... Configuration > Change from "High Fidelity Playback (A2DP Sink)" to "Handsfree Head Unit (HFP)" > Background (or maybe ANC) in the earphones changes > Test > Nothing (not even external speakers)
Any clever suggestions? 🤔