The Medieval Period lasted roughly 1,000 years. The way that people spoke in the Early Middle Ages was very, very different from the way that they spoke in the Late Middle Ages. That would go for people living in any part of the world, but assuming that you're asking about England in particular, the changes were quite drastic. Old English was spoken up until the Norman Conquest, at which point it began to morph into what we call Middle English. Standardisation would not enter into things until much later. Even in Shakespeare's day it was quite possible to pronounce and to spell the same word in several different ways and a variety of grammatical forms were in use simultaneously. Depending on which specific year your story is set, as well as where in England and who in particular happens to be speaking, there are likely a few ways of rendering speech from that era. To say that one approach would be more accurate than the next would require some research.