Plant Pronunciation from the fine Gardening website is currently down.  It looks like their working on improving it and it should be up soon!  The links/photos below should bring you there once their back up and running!  

In the meantime, this site from OverPlanted should help.

(although it's not an audio guide)

Here's another one from OverPlanted!:

Overplanted - Botanical Latin Pronunciation Guide 

Gardening magazine online has a plant pronunciation section that is very cool. 

   Here's the link below, have fun!