The Shulchan Aruch (Orach Chaim 62:3) rules that one fulfills the mitzvah of Keri'at Shema even if he did not hear the words that he has said--with one condition:
צריך להשמיע לאזנו מה שמוציא בפיו, ואם לא השמיע לאזנו, יצא ובלבד שיוציא בשפתיו
Regarding the bolded words, I see two ways to understand them: either, you must move your lips without actually emitting any sound, and that is enough (bedi'avad), or you must actually pronounce the words, and then even if your words did not reach your ears (for example, you are in a noisy room) that is enough; but merely "mouthing" the words would not be yotzei.
Are there any sources or proofs one way or the other?