First off, you are assuming this is a SoundGroup issue or a Gaia issue. It is not. You should load your mp3 on your own and see that you get no ID3 data.
The reason is most likely because the ID3 data is a version that is incompatible with Flash. One example of this is anything tagged by iTunes.
Flash only loads outdated ID3 formats. It's bullshit, if you ask me. The workaround is to use Ben Stucki's metaphile classes, which use a combination of URLLoader and ByteArray to read the id3 data from the header bytes of your sound file.
http://github.com/benstucki/metaphileAnother solution is to put the id3 information in your xml. I know that isn't ideal, but you can yell at Adobe for their weak support of ID3.