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? 🤔
If you have Pulseaudio it is answered here Ubuntu 20.04 no sound out of Bluetooth headphones. PulseAudio Volume Control can be installed from Software in Ubuntu. After installation choose Output Devices.
In future pls. add all info relevant to your question, in this case what version of ubuntu you are having problem of, if you have pulse audio installed. and also read this https://askubuntu.com/help/asking