For one that uses the built in Ubuntu NotifyOSD system (could easily hack to add sound to it):
sudo add-apt-repository ppa:sect2k/zimdicator
sudo apt-get install zimdicator
or grab it via:
http://launchpad.net/zimdicator
nohub zimdicator.py (if it doesn't start automatically/don't remember)
configure connection settings via system > preferences > zimbra mail indicator (or zimdicator-config.py)
or there's another linux one:
ZimbraNotify | Zimbra :: Gallery