I heard that for Written Torah you need to SAY the words, and for oral Torah you need to understand but not necessarily say.
However, the Baal HaTanya is extremely stringent and says that meditation does not count as speech, and that you are not mekayem the mitzvah of talmud torah unless you articulate the words with your mouth. However, in chapter 5 of the Tanya, he seems to contradict himself and says that knowing and understanding Torah is an even bigger mitzvah than saying it. One way to resolve the contradiction is to say that one fulfills the mitzvah ONLY if he articulates the words (e.g., he reads the Chumash out loud). An even bigger mitzvah would be to UNDERSTAND the words AND to say them. Simply understanding without saying is not a mitzvah d’oraysa of talmud torah, though it is certainly not worthless – since in another place, the Baal HaTanya says in the name of the Arizal that when a Jew meditates silently on the narratives in the Written Torah (and presumably on any other matter of Torah, as well), his soul becomes bound up with Supernal Wisdom.
Many times, I see men on the bus or train learning gemara without articulating the words? Is this wrong? is it assur to learn out loud in these kinds of places b/c of immodestly dressed women, so they have no choice but to learn silently – but otherwise one MUST learn out loud?