1. One must let his ears hear the words that he says with his mouth when saying the Shema and its blessings. If he did not say it loud enough for his ears to hear, he has still fulfilled his obligation so long as he enunciated the words with his lips. If, however, he only thought the words in his mind, he has not fulfilled his obligation. (Shulchan Aruch and Mishne Berurah 61)
2. A person who is ill or unable to say the Shema for another reason, may think in his heart, and Hashem will send him reward for this. He has nevertheless not fulfilled his duty, and if he gets the chance to say it again before the final time for the Shema on that day, he must do so. (ibid)
