The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Balak (2; listen to audio recording for precise citation), writes that when one recites a Beracha, he must do so slowly, carefully, with concentration, and loudly enough that he can hear the recitation. He sharply condemns the "Burim" (ignoramuses) who recite Berachot quickly and carelessly, such that they mispronounce the text, omitting letters and at times even complete words. The Ben Ish Hai emphasizes that if a person omits even a single word from the phrase "Hashem Elokenu Melech Ha’olam," he has not fulfilled his obligation. Hacham Ovadia Yosef rules a bit more leniently, claiming that if a person omitted the word "Ha’olam" he has nevertheless fulfilled his obligation. In any event, one must certainly make a point of reciting every Beracha slowly and carefully, and ensure to recite each word properly. Often, people recite the first part of the Beracha very rapidly and end up saying gibberish, like, "Baruchtanai Elmlacholam." As we have seen, one does not fulfill his obligation by reciting a Beracha in this manner.
As the Ben Ish Hai mentioned, one should also ensure to recite Berachot audibly. Among the reasons given for this Halacha is the fact that one should allow the people around him the opportunity to answer "Amen." The Sages teach, "Gadol Ha’one Yoter Min Ha’mebarech" – the one who answers "Amen" performs a greater Misva than the person who recites the Beracha. The recitation of a Beracha serves as testimony to the idea expressed in the Beracha. For example, when a person recites the Beracha of "She’hakol," he testifies to the fact that everything in the world was created in accordance with the will of God. Testimony in Jewish law is valid only when it is presented by two or more individuals; one person’s testimony does not suffice as valid legal testimony. Therefore, when a person answers "Amen" and thus joins the person who recited the Beracha in issuing the given statement, he transforms the Beracha into valid testimony, and his reward is thus greater.
It emerges, then, that when a person recites a Beracha aloud, he performs two Misvot: he recites a Beracha, and he allows those around him to fulfill the great Misva of answering "Amen." And even if a person is alone, he should still ensure to recite the Beracha aloud, since it may turn out that somebody else is, in fact, within hearing distance. Moreover, even if a person is confident that there is nobody in the vicinity, it is written that the angels in the heavens can hear our Berachot and answer "Amen." Under all circumstances, then, one should recite Berachot in a loud, audible voice.
Summary: One who recites a Beracha must do so slowly, ensuring to pronounce each and every word properly, as mispronouncing a word could invalidate the Beracha. One should also concentrate on the meaning of the words, and recite the Beracha in an audible voice, even if there is nobody around him to answer "Amen."
