One must dedicate one's voice and words to holiness. One's mouth is an organ of Malchut. (The Sefirah of Malchut, Kingship) One's speech can be a revelation of Malchut. (cf. Likkutei Halachot, Choshen Mishpat, Apotrofus 3:13)
- from Sefirot by Y David Shulman pg 227
Speech in general is connected to Malchut. One fulfills the commandment of learning Torah only when one learns aloud, drawing the transcendent ideas of Torah into the vessels of speech. Similarly, one fulfills the act of prayer only when one pronounces the words, providing a recepatcle for one's yearning and for receiving G-d's reply.
Because speech is Malchut, the final sefirah, it is surrounded by unholy forces. That is why it is so important to guard one's speech from bad language, gossip and falsehood (cf. ibid, Choshen Mishpat, Halvaah 1:4) One must counter those voices of ugliness with strong, brazen expression of holiness, with a strong, striking speech (cf. Ibid Choshen Mishpat, Maakah Ushmirat Nefexh 4:11) Speech of this kind shines on one and brings to fruition the verse, "The world is filled with His glory." (Isa. cf. Likkutei Halachot, Yoreh Deah, Ever Min Hachai 2:1)