For most of our relevant period, Middle English is the form of English that would have been used ("Old English" was more Germanic and spoken prior to somewhere around 1100 AD).
Middle English actually resembled modern English quite a bit, but with some strong differences in pronunciation. Most of our modern spellings derive from this time (even though the spellings of the time also differed quite a lot). Most of the silent letters in modern English were pronounced back then. For instance, the "k" in "knight" was pronounced (and so was the "gh" like a soft form of the "ch" in the German "ich", and the "i" had more of an "ee" sound). Ironically, as a result, the text is probably easier to read than it would be to listen to it spoken.
I just thought I'd share since I ran across some of this in some random googling. It's useful to understand if you look at the untranslated versions of Chaucer.
Here's a neat little succinct description, though the links to the audio samples are broken:
http://webpages.marshall.edu/~will2/chaucer.htmlA youtube sample of middle English:
[youtube]http://www.youtube.com/watch?v=QE0MtENfOMU[/youtube]