I right click on a song, click View metadata, and it says at the top "Metadata for the selected item cannot be edited". I am sharing via samba and have it set to allow anynomous access. Does Songbird not allow this by default for music being streamed to it?
If you are using Samba to share files in a selected folder check the smb.conf definitions.
Go to "Share Definitions" and before [Printers] check the parameter "writable": if it's set to "no" you cannot edit any file into the shared folder...