Sharing MP3s between streams is not supported directly by Centova Cast, but you can achieve the same functionality by creating a symbolic link in the stream's var/spool/media/ directory which points a shared directory.  So for example, you might do this:

1. Put all of your shared MP3 files in the folder:
   /home/centovacast/sharedmp3s/
2. Create a symlink by running:
   ln -s /home/centovacast/sharedmp3s /home/centovacast/vhosts/USERNAME/var/spool/media/shared

Then repeat step 2 for each additional stream that should use the shared MP3 directory.  You should of course replace USERNAME with the username for each stream.

Note that if you use Centova Cast's FTP integration features, all users sharing the MP3 directory will have access to upload, overwrite, and delete the shared MP3 files.  If this is a problem, you may wish to use UNIX file permissions to "lock down" the existing MP3s and prevent modifications.