Flash MX 2004.
frame.soundName
Property; a string that specifies the name of a sound that is attached directly to a frame (frame.soundLibraryItem), as stored in the library.
The following example changes the soundName property of the first frame to "song1.mp3"; song1.mp3 must exist in the library:
fl.getDocumentDOM().getTimeline().layers[0].frames[0].soundName = "song1.mp3";