The good news is you can play .mp3 with Safari (at least on MacOS).
The issue appears here : Babylon.js/audioEngine.ts at 1d60be346cf68274e5dd22ee91cdadb5e8b2949f · BabylonJS/Babylon.js · GitHub
Where the check is false and isMP3supported never gets true.
If I force it to be true, the sound can be heard.