There are a number of very interesting articles on the Web (including a lecture on Youtube) - regarding the Great Vowel Shift which was a massive sound change affecting the long vowels of English during the fifteenth to eighteenth centuries.

http://www.en.wikipedia.org/wiki/Great_Vowel_Shift

There is also an excellent website produced by the Furman University on this subject, but just can't see to get the link correct.