Great is Torah... [which] is acquired with forty-eight qualities. They are the following: with [constant] study, with attentive listening, with verbal enunciation, with perception of the heart...
We possess the power of speech so that we can communicate with others. Speech thus embodies transcendence of self, since its primary purpose is not for the self but for the other. It is therefore associated with the soul's essence, since it is the soul's infinite essence that enables and finds expression in self-transcendence.
[We therefore find that] although speech seems to be inferior to the intellect and emotions it articulates, it is capable of influencing them. This is because of speech's root in the soul's essence.
Hence the importance of speaking words of Torah aloud, since by doing so we enhance our understanding of the Torah subject we are studying. Similarly, we recite the words of prayer aloud, since the act of speaking enhances our emotional feelings during prayer.
Furthermore, speaking words of Torah forces us to utilize and thereby sanctify our physical selves. Forgetting Torah then becomes less likely, since forgetfulness stems from the weakness of the physical self.
Moreover, by speaking words of Torah, we fulfill purpose in this world, which is to elevate and sanctify physical reality.
Holtzberg Memorial Edition