Which is the proper way to recite the Amida silently, or audibly?
The Shulchan Aruch writes (101) that when praying the Amida one must move his lips and enunciate the words; thinking the words in one's mind does not fulfill the obligation, as indicating in the verse describing the prayer of Chana, mother of the prophet Shemuel: "Only her lips were moving
" (Shemuel I 1:13). This ruling of the Shulchan Aruch is shared by all authorities.
There is, however, disagreement among the authorities as to how loudly the Amida should be recited. The Shulchan Aruch rules that one should recite the Amida softly enough so that those standing near him will not hear his prayer, but loudly enough to allow him to hear his own prayer. Among the Kabbalists, however, we find different traditions in this regard. The Chid"a (Rabbi Chayim Yosef David Azulai, Israel, 1724-1806), in his work "Birkei Yosef" (a commentary to the Shulchan Aruch), records a tradition he heard in the name of students of the Arizal (famed Kabbalist, Israel, 1534-1572) that the Amida must be prayed silently. If one's prayer is even slightly audible, the "Chitzonim" (harmful spiritual forces) are capable of disrupting the prayer's efficacy and preventing it from reaching its destination. By contrast, the Siddur Beit Oveid (in Hilchot Tefila, 75), after recording these comments of the Chid"a, cites a ruling from the Kabbalist Mahari Seruk, a student of the Arizal, that one must pray loudly enough to hear his own prayer. Thus, different traditions exist as to the proper way to recite the Amida according to the Kabbalah.
The Ben Ish Chai (Rabbi Yosef Chayim of Baghdad, 1833-1909), in his work "Od Yosef Chai" (Parashat Mishpatim, 3; listen to audio for precise citation), rules that the Halacha on this issue depends on the individual's ability to properly pronounce the words and concentrate on his prayer. One who feels that he can accurately enunciate the words and pray with concentration reciting the Amida inaudibly, then he should do so, in accordance with the tradition cited by the Chid"a. If, however, one suspects that he might swallow his words or experience difficulty concentrating unless he recites the Amida audibly, then he should follow the Shulchan Aruch's ruling and pray the Amida loudly enough to hear his words. The Ben Ish Chai adds that his father followed the practice to recite the Amida audibly.
Chacham Ovadia Yosef, in his work Halichot Olam (vol. 1, p. 157), writes that the Halacha follows the position of the Shulchan Aruch, that the Amida should be recited audibly. This will enhance his concentration and ability to pronounce the words properly.
Of course, those who recite the Amida audibly must ensure to recite it softly enough that only they and nobody else in the synagogue can hear their prayer.
Summary: According to all authorities, one must recite the Amida with his lips, and not merely think the words in his mind. One should recite the Amida audibly, such that he but nobody else can hear his prayer. If he feels capable of properly enunciating the words and concentrating while praying silently, the Ben Ish Chai says to pray silently.
