does anyone know if this is possible?
Using the native flash sound class, you can read the id3 metadata property of the sound object, however this depends on the software that encoded the mp3 putting the encoding information in the metadata. there's nothing inherent to the mp3 itself that reports this to the sound class, and the metadata is often missing or unreliable.
<a href="http://livedocs.adobe.com/flash/9.0/ActionScriptLangRefV3/flash/media/Sound.html#id3" rel="nofollow">http://livedocs.adobe.com/flash/9.0/ActionScriptLangRefV3/flash/media/Sound.html#id3</a>
alternatively there's this 3rd party utility for reading actual mp3 header information, which is much more accurate.
<a href="http://code.google.com/p/mp3infoutil/" rel="nofollow">http://code.google.com/p/mp3infoutil/</a>