Om Mani Padme Hum is the mantra of Chenrezig.
One way to pronounce it is: Aum Mah-Nee Pah-May Hum
A loose translation into contemporary english could be: Ahh, jewel, lotus, hmmm.
You can get a hear the mantra at the Enlightenment Page.
* Jewel ----- In ----- Lotus *
* Body ----- Speech ----- Mind *