registerAudioEngineBackend function Audio
Registers factory under backend id, replacing any existing entry.
A document's audioEngine component names its backend by id; backend
packages register their factory at app startup
(registerFmodAudioBackend()) so documents authored against them realize.
flutter_scene itself ships no backend, so an app without one skips the
component.
Implementation
void registerAudioEngineBackend(String id, AudioEngineBackendFactory factory) {
_audioBackends[id] = factory;
}