Flash MX 2004.
soundItem.bitRate
Property; a string that specifies the bit rate of a sound in the library. This property is available only for the MP3 compression type. Acceptable values are "8 kbps", "16 kbps", "20 kbps", "24 kbps", "32 kbps", "48 kbps", "56 kbps", "64 kbps", "80 kbps", "112 kbps", "128 kbps", and "160 kbps". Stereo sounds exported at 8 Kbps or 16 Kbps are converted to mono. The property is undefined for other compression types.
If you want to specify a value for this property, set soundItem.useImportedMP3Quality to false.
The following example displays the bitRate value in the Output panel if the specified item in the library has the MP3 compression type:
alert(fl.getDocumentDOM().library.items[0].bitRate);