1) Even when saying the verses of praise one should not raise one’s voice too much, as Hashem hears even when one speaks quietly. (Shulchan Aruch and Mishne Berurah 101)
2) However, the cantors who when repeating the Amidah raise their voices so that their voices should be easily heard are acting correctly as the main reason for the institution of the repetition of the Amidah is so that the congregation should hear it. Nevertheless, they should intend to show off their own nice voices. (ibid)