It is reported that the Arizal made a point of reciting all the prayers quietly, without raising his voice – even the chapters of Tehillim recited in the Pesukeh De'zimra section. He felt that praying in a low, subdued voice reflected awe, fear and humility, which is the appropriate manner in which to stand in prayer before the Almighty. On Shabbat, he raised his voice somewhat during the prayers as an expression of honor for the special sanctity of Shabbat.
Nevertheless, there are those who recite the prayers (besides the Amida, which must be recited quietly) out loud and with great emotion and enthusiasm. Hacham Ovadia Yosef writes that this is allowed if this helps a person pray with greater Kavana (concentration). Ultimately, he says, a person should pray in a manner that helps him serve G-d to the best of his ability.
However, those who pray out loud to enhance their Kavana must ensure that their prayers do not disturb the people around them, for if they do, then the benefits of their Kavana are offset by the harm caused to other people's prayers.