Ĥazanim must have kavana that their singing is the sake of heaven. If they prolong their ĥazanut (cantorial virtuosity) with the sole purpose of showing off their beautiful voices, the Torah states about them, “They have raised their voice against Me, so I hated them” (Yirmiyahu 12:8). They are using the holy prayer service to self-aggrandize. Even one whose sole intent is for the sake of heaven should not prolong his singing, so as not to burden the congregation (Rashba; SA 53:11).
Ĥazanim may not repeat the words of berakhot and Kaddish because doing so changes the formula that the Sages established. If repeating the words alters the meaning of the berakha, those words are considered a hefsek, and the ĥazan must recite the berakha again from the beginning. However, if there is no change in the meaning, be-di’avad he does not need to recite the berakha again, because he did not interrupt its recitation with an extraneous matter (see Igrot Moshe OĤ 2:22; Yabi’a Omer 6:7).
The poskim disagree about whether one may use melodies of profane songs for prayers and liturgy. In practice, when the congregation is not familiar with the crude lyrics of the song, it is customary to be lenient and fit the melody to the prayer. However, if the congregation recognizes the song, one may not use that tune for prayer, because when the people sing it, they remember the crude theme of the song and their kavana is likely to be disrupted (Peninei Halakha: Prayer, ch. 4 n. 1). It is forbidden to appoint a singer who normally sings songs with vulgar content to be ĥazan (Rema 53:25).
It is not fitting for one to desire to be ĥazan. Therefore, when the gabbai asks someone in the congregation to lead the service, proper protocol is do refuse at first, though he should not decline excessively (see SA 53:16; Peninei Halakha: Prayer 4:3). If one is capable of leading the prayers but refuses, he indeed insults the honor of the prayer service as well as God’s honor. In particular, one who God endowed with a talent for singing and a pleasant voice should not decline on Shabbat and festivals, since the prayers on those days are rich with song and melody. If he refuses to pray out of stubbornness or laziness, and does not praise God with his voice, it would have been better had he not come into this world (Sefer Ĥasidim 768). The Sages say about Navot of Jezreel that he had an exceptionally pleasant voice and would ascend to the Temple in Jerusalem for the three pilgrimage festivals, where all Israel would gather to hear him. The one time he stayed home to guard his vineyard, he was punished; lawless people testified that he rebelled against the king and he was put to death (Pesikta Rabbati §25).