One plugin that Squirrelmail has that I really like is when you received a new message a .WAV file will play.
Since RoundCube uses Ajax to refresh and check for new messages, it could be fairly simple, for some one familiar with the script, to implement a call to a sound file if there are new messages.
There are already PHP addons available for scripts such as PHPforums/Nuke and Vbulletin and I can provide links if it will help.