Some fragments of answers I found in the forum itself, e.g.
360 Video Dome Video Caching? - Questions - Babylon.js (babylonjs.com)
Issue with 360° videos: 360 Video Player with Youtube Videos - Questions - Babylon.js (babylonjs.com)
You can attach the speaker-object to the camera as a parent element, so it is always visible.
meshSpeaker.position.z = 10; // play around with the x,y,z position to geth the speaker in the scope of the camera
meshSpeaker.parent = camera;