Movieclips on timeline disappearing = to be disposed by GC?

Lately I’ve been reading up on how to ensure your objects in Flash are properly prepared to be disposed by the GC. But most of the information seems to be with regards to programmatically-created objects, or at least objects with a reference in Actionscript.:pa:

What about objects that are dragged onto the timeline in Flash? They don’t have an instance name, aren’t referenced anywhere, and disappear when the playhead moves to another frame. Are they thus considered to be totally unrefenceable, and “disposed”?

It seems to be that way, but I remember there was something about sound playback looping infinitely when the playhead keeps starting the sound playback, and yet the sound is also looping on its own. So I’m not so sure now.

And what if a no-name movieclip A, contains an Actionscripted movieclip B within itself, will it ever be cleaned up when the playhead moves past it?