Not sure of a specific  website that teaches,  but the organization   called The Riggs Institute  teaches the "Spalding Method"  of Phonetics.  That is the final word on the science.

http://www.wiktionary.org/

Thanks a lot!! I'm going to have a look to this webs.

Greetings.

Several online dictionaries will speak the word for you.  

You might try 

http://www.spanishdict.com/

http://www.merriam-webster.com/

http://www.thefreedictionary.com/

In addition, a site that I like produces flashcards and pronounces the words on the flashcards in your language(s) of choice.  Here is the link:  http://quizlet.com/latest

I have been using a online dictionary called Kingsoft PowerWord, which is a very popular in China, to learn vocabulary, because it is not only a dictionary, but also has the pronunciation function, even contains vocabulary lists for different English level examinations. 

The online dictionary really helps me a lot for learning the pronunciation of words. Maybe you can get one if you want a try.