Flash MX 2004.
frame.soundEffect
Property; a string that specifies effects for a sound that is attached directly to a frame (frame.soundLibraryItem). Acceptable values are "none", "left channel", "right channel", "fade left to right", "fade right to left", "fade in", "fade out", and "custom".
The following example specifies that the sound attached to the first frame should fade in:
fl.getDocumentDOM().getTimeline().layers[0].frames[0].soundEffect = "fade in";