This simple plugin uses pynotify to notify on song changes in sonata.
This is when you don’t have the album cover:
This one shows a preview of plugin when option “Album art” is checked (and mp3 file has some decent id3-tags):
You can download it using bzr:
bzr branch lp:~l3on/sonata/sonata-plugins
Then copy notify.py into the plugins directory of sonata.
For a better result, uncheck: Sonata → Preferences → Extra → Popup notification on song changes