The Gemara in Masechet Eruvin discusses the importance of studying Torah verbally, rather than simply reading the material with one’s eyes. The Gemara explains that in order to retain knowledge, one must ensure that the information is absorbed throughout his body, which one accomplishes by vocalizing the words as he studies. The Shulchan Aruch Ha’Rav written by Rav Shneur Zalman of Liadi, first Rebbe of Lubavitch (a”h), goes so far as to say that one who learns silently has not fulfilled the obligation of Torah learning at all. Since this obligation is presented in the Torah with the words, “V’dibarta Bam,” it demands that one actually speak in matters of Torah, rather than just read them.
The Pele Yo’etz (a”h) elaborates and states that the sound of Torah studied out loud and articulately “pierces the heavens” and “brings joy to Hashem.” Those who study silently or inarticulately forfeit a great opportunity to accrue reward and bring joy and pleasure to Hashem. Likewise, the Magid Mesharim (a sefert that documents the information revealed to Rav Yosef Karo a”h through an angel) mentions that the audible sound of Torah learning creates many angels that protect and assist a person.
Rav Baruch Ben Chaim (a”h), would enter the Bet Midrash and see the students learning silently. He would comment, “This room is filled with Torah!” The students would initially feel gratified over having received such a compliment, but he would then explain, “The room is filled with Torah – because it is not ascending to the heavens!” Only when one verbalizes it out loud, does it rise to the heavens and please Hashem.
Therefore, one must try to always learn Torah aloud, rather than silently, even when he learns alone or in private.