There are a number of excellent online dictionaries that provide not only the definitions of words, but also sound files to hear the pronunciation of the words you look up. Here are a few that are worth exploring:
Vocabulary.com - http://www.vocabulary.com/
This site offers vocabulary quizzes and a dictionary with easy-to-understand definitions, sound files for word pronunciation and the possibility of creating your own vocabulary lists. The latter feature is available to registered users. Registration is free.
Wordsmyth Educational Dictionary and Thesaurus – http://www.wordsmyth.net/
On this page, students or teachers can look up any word in the online dictionary/thesaurus and discover the definition and the pronunciation of the word. As well they can browse the words alphabetically around their chosen word, and see entries that contain their chosen word. Definitions come with images and animations, lists of synonyms, derivations, related words, and examples of use in sentences. There are three versions: A beginner’s dictionary, a children’s dictionary, and an advanced dictionary.
Dictionary.com – http://dictionary.reference.com/
Dictionary.com is a multi-source dictionary search service produced by Lexico Publishing Group, LLC.To use the dictionary, simply type a word in the search box and the service will perform a search for the word in the several dictionaries hosted on the site.