Flash MX 2004.
frame.soundLibraryItem
Property; a library item (see SoundItem object) used to create a sound. The sound is attached directly to the frame.
The following example assigns the first item in the library to the soundLibraryItem property of the first frame:
// The first item in the library must be a sound object.
fl.getDocumentDOM().getTimeline().layers[0].frames[0].soundLibraryItem
=fl.getDocumentDOM().library.items[0];