old english did not sound like modern english, which differs widely in phonology and especially in grammar, so taht old english is unintelligible to modern speakers.
the average english was strongly influenced by the anglo-norman as later by the anglo-french. the pronunciation was affected by the norman with the sounds "/ θ / and / ð / " transcribed as th.
modern english was standarized from the london dialect and extended by the government and administartion as well as by the effects of printing, began to acquire more words from other languages as well to imporve in a more fluent pronunciation.