Audio sub-system failed to initialize.
Audio sub-system is ready; all embedded tracks have been registered.
Audio sub-system is initializing for the current media (start of setup).
An individual sidecar audio track failed to load.
An individual sidecar audio track finished loading successfully.
An individual sidecar audio track has started loading.
The active audio track has changed.
An individual sidecar audio track has been unloaded.
The set of tracks requesting media buffering has changed (a track started or stopped requesting buffering).
Audio sub-system has been torn down and all tracks removed.
Fired after any change in audio sub-system (catch-all change notification).