Gemara

Sotah 32a (Mishnah): Kri'as Shema may be recited in any language.

32b: We expound "Shema" - in any language you hear (i.e. understand).

(Beraisa - Rebbi): Shema Yisrael must be said as it is written (i.e. in Hebrew);

Rebbi learns from "Shema" that one must say the words audibly (to his ears).

Chachamim hold that even if one does not say it audibly, he fulfills the Mitzvah.

Berachos 15a (Mishnah): If one said Keri'as Shema inaudibly, he was Yotzei;

R. Yosi says, he was not Yotzei.

15b (Beraisa - R. Yehudah citing R. Elazar): One must say Shema audibly -- "Shema Yisrael..."

R. Meir: It says "Al Levavecha" -- intent suffices!

(Rav Chisda): The Halachah follows R. Yehudah in the name of R. Elazar, and the Halachah follows R. Yehudah (the first Tana of the Mishnah 15a).

Had he taught only that the Halachah follows R. Yehudah, one might have thought that even l'Chatchilah it need not be audible. Had he taught only that the Halachah follows R. Elazar, one might have thought that even b'Di'eved it must be audible.

Rav Yosef taught that they argue about Keri'as Shema, for it says "Shema Yisrael" but regarding other Mitzvos, all agree that he was Yotzei.

20b (Mishnah): A Ba'al Keri (he may not say Divrei Torah or pray until he immerses from his seminal emission) must think in his mind the Berachos before and after Keri'as Shema, but he does not say them. He blesses (Birkas ha'Mazon) after eating, but only thinks in his mind the Berachah before eating.

(Gemara - Ravina): Our Mishnah teaches that Hirhur (thought) is k'Dibur (like speech). If not, there would be reason to have Hirhur!

(Rav Chisda): Hirhur is not like Dibur. If it were, he should say the words!

Question: If it is not like speech, there is no reason to have Hirhur!

Answer (R. Elazar): He should not be idle while everyone else is saying Shema.

Shabbos 150a (R. Yochanan): R. Yehoshua ben Korchah expounds "Mi'Mtzo Cheftzecha v'Daber Davar" - speech (that explicitly discusses Melachah) is forbidden on Shabbos. Thoughts are permitted.

Question (Rav Acha bar Rav Huna): This implies that R. Yochanan holds that Hirhur Lav k'Dibur. Why does he forbid thinking about Torah in a bathhouse, due to "v'Hayah Machanecha Kodesh"? He should say that "v'Lo Yir'eh Vecha Ervas Davar" forbids only Dibur!

Answer: "Ervas Davar" forbids reciting Shma facing a naked Nochri.

Rishonim

Rif (Berachos 8b): If one said Shema inaudibly, he was Yotzei b'Di'eved.

Rambam (Hilchos Kri'as Shema 2:8): One must say the words of Shema audibly. If he did not, he was Yotzei.

R. Yonah (Berachos 12a DH Masnisin): The Ge'onim rule like Rav Chisda, who say that Hirhur Lav k'Dibur. Also R. Yochanan holds that Hirhur Lav k'Dibur. Only regarding filth, the Torah was stringent about thoughts of Torah. We must say that the opinion that one is Yotzei Birkas ha'Mazon in his heart means that he said the words with his lips, but inaudibly.

Rosh (3:14): We concluded that Hirhur Lav k'Dibur, and a Ba'al Keri has Hirhur so he will not be idle while everyone else is saying Shema.

R. Yonah (8b DH Gemara): If one must say a Berachah and his hands are not clean or the place is not clean, e.g. a sick person's bed, he thinks the Berachah in his heart, and Hash-m rewards him for his intent, even though Hirhur Lav k'Dibur.

Question (Beis Yosef 85 DH Lo): One may not think about Torah in a filthy place! Perhaps he discusses when his hands or the place is not truly filthy. They are dirty, but there is no Tinuf (e.g. excrement). If not for Ones, he would be obligated to clean the place and say the Berachah audibly.

Poskim

Shulchan Aruch (OC 61:9): Some have the custom to say Shema loudly. Some say it quietly.

Magen Avraham (16): Ha'Ram Alashkar (10) says in the name of Rav Hai Gaon that we do not say it loudly, lest someone rely on hearing it and not say it himself. The Re'em (1:41) says that one can be Yotzei in this way.

Kaf ha'Chayim (6): If one who does not know how to say Shema hears it in a Minyan, ha'Ram Alashkar agrees that he is Yotzei through hearing. However, some say that one is Yotzei through the first verse, and everyone knows this. Therefore, if there is a Minyan he should say the first verse and hear the rest. If there is no Minyan, he repeats word for word what he hears.

Shulchan Aruch (63:3): One must say the words of Shema audibly to his ears. If he did not, he was Yotzei, as long as he said the words with his lips.

Mishnah Berurah (4): Mid'Rabanan, one must say it audibly. The Ra'avad requires this mid'Oraisa.

Shulchan Aruch (4): If due to illness or another Ones one said Shema in his heart, he was Yotzei.

Source (Beis Yosef DH Kasuv b'Orchos, citing Orchos Chayim): The Yerushalmi (Berachos 1:1) learns this from "Imru bi'Lvavchem."

Mishnah Berurah (7): He was not truly Yotzei, for Hirhur Lav k'Dibur. When the Ones passes, if it is still time for Shema, he should repeat Shema.

Kaf ha'Chayim (8): Some say that he need not repeat it, for in a case of Ones, Hirhur k'Dibur.

Rema (4): If a person is in a place that is not totally clean and he is unable to clean it, he should think Shema in his heart.

Magen Avraham (2): The middle room of a bathhouse is such a place.

Taz (1): Likewise, if someone woke up at night and wants to drink but cannot wash his hands, he should think the Berachah in his heart.

Rebuttal (Mishnah Berurah 8, citing Mateh Yehudah): He should get up and wash. If this is impossible, he should clean his hands on the wall or anything else. This suffices even if he touched covered parts of the body.

Rema: If it is utterly filthy he may not think about Divrei Torah.

Mishnah Berurah (9): If one must say Shema or a Berachah but may not, he should think about his obligation and feel bad that he is unable to fulfill it