According to the Shulchan Aruch Harav (Hilchos Talmud Torah ch. 2:12).
וכל אדם צריך ליזהר להוציא בשפתיו ולהשמיע לאזניו כל מה
שלומד בין במקרא משנה ותלמוד אא"כ בשעת עיון להבין דבר
מתוך דבר. וכל מה שלומד בהרהור לבד ואפשר לו להוציא בשפתיו
ואינו מוציא אינו יוצא (ברכות *ב׳ ע״ש בפ״י ד׳ ט״ו) בלימוד זה
י״ח מצות *ולמדתם אותם וכמ״ש לא ימוש ספר התורה הזה מפיך
והגית בו וגו׳. וכמו בכל המצות התלויות בדבור *שאינו יוצא בהן
י״ח בהרהור אא״כ שומע מפי המדבר *שהשומע כעונה בפיו. אך אם
מוציא בשפתיו אף על פי שאינו מבין אפי׳ פירוש המלות מפני שהוא
ע״ה ה״ז מקיים מצות ולמדתם. ולפיכך כל ע״ה מברך ברכת התורה
בשחר *לפני הפסוקים. וכן *כשעולה לס״ת.
Everyone must take care to say with their lips and hear with his ears whatever he learns, whether it be Psukim, Mishna or Gemara, unless he's learning in depth. And whenever one learns quietly and doesn't say his learning out loud doesn't fulfill his obligation to "learn it" ... Like any other commandments which are dependent with speech, one doesn't fulfill his obligation through thought, but has to hear it from someone else.
But if one says the words and doesn't understand them, he fulfilled his obligations 
His source is the Gemara in Berachos Berachos 15b which says
מיתיבי לא יברך אדם ברכת המזון בלבו ואם בירך יצא אלא אי אתמר הכי אתמר אמר רב יוסף מחלוקת בק"ש דכתיב שמע ישראל אבל בשאר מצות דברי הכל יצא והכתיב הסכת ושמע ישראל ההוא בדברי תורה כתיב:
The Gemara objects based on what was taught in a baraita: One may not recite the Grace after Meals in his heart, inaudibly, and if he recited the blessing in that manner, he fulfilled his obligation. In this example of the rest of the mitzvot, the obligation to hear the recitation of the blessing is only ab initio. Rather, Rav Yosef’s statement must be emended. If this was said, it was said as follows; Rav Yosef said: The dispute as to whether or not a deaf person fulfills his obligation is only in the case of the recitation of Shema, as it is written: “Hear, Israel.” But regarding the rest of the mitzvot, all agree that a deaf-mute fulfills his obligation. The Gemara asks: Isn’t it written: “Pay attention, and hear, Israel”? The Gemara responds: That verse is written with regard to matters of Torah; one must pay close attention to what is written in the Torah.
Which implies that Shema Yisrael (implying one has to read it out loud) apply more to Torah study than to Blessings (about which the Gemara debates earlier).
So too, the Shev Yaakov says that since the commandment of Learning Torah comes from "Veshinantam Levanecha" - teaching it to one's son, the only way to fulfill the Mitzvah is in a way that a hypothetical son would be able to understand, and must be done audibly.
Also, the Shulchan Aruch (47:4) and Chayey Adam say that one doesn't say blessings on the Torah if one just thinks Torah thoughts (though the Gra does ask there that Higisa implies thought, not speech, and the Chayey Adam says that the Gra leaned toward saying Birkas HaTorah on thought), implying that one doesn't perform the commandment of Torah study through thought .
So too, Rabbi Braun explicitly says that one does not fulfill his obligation of learning Torah through hearing tapes. Though he writes that there are some opinions who say that silent reading from books isn't considered "thinking", the Shulchan Aruch HaRav and the Rama Orach Chaim 67 (Which says that one is allowed to silently read from Seforim when he is holding in a place where one is not allowed to interrupt. If silently reading from a text is considered speech, then reading would be the same hefsek as speaking).
 The footnote there says that this is a typo.
 The next Syif says that this is only true about the written Torah. When one learns from the Oral Torah one doesn't fulfill his obligation unless he understands what he says.
 Though the Shaagas Aryeh Siman 24 implies that while one fulfills the Mitzvah of Torah study through thought, one can't say the blessing over it since the source-verse for the blessing is "Ki Shem Hashem Ekrah", implying that there must be words spoken.
 Though not recommended, as he may absentmindedly start reading out loud.