Flash MX 2004.
bitmapItem.quality
Property; an integer that specifies the quality of the bitmap. To use the default document quality, specify -1; otherwise, specify an integer from 0 to 100. Available only for JPEG compression.
The following code sets the quality property of the first item in the library of the current document to 65:
fl.getDocumentDOM().library.items[0].quality = 65;
alert(fl.getDocumentDOM().library.items[0].quality);