While singing, the chazanim must have kavanah for the sake of Heaven, but if they prolong their chazanut (cantillation) and their only intention is to show off their beautiful voices, the Torah writes about them, “It raises its voice against Me, therefore I have hated it” (Jeremiah 12:8). They are using the holy prayer service to arrogantly boast as if on a stage. Even a person whose only intent is for the sake of Heaven should not excessively extend his cantillation so as not to burden the congregation (Rashba; Shulchan Aruch 53:11).
While chanting the prayers, the chazanim are prohibited from repeating any words of the berachot and Kaddish, because doing so changes the nusach that the Chachamim established. If the repetition of the words alters the meaning of the berachah, those words are considered to be an interruption (hefsek) and the chazan must recite the berachah again from the beginning. However, if the meaning does not change, b’dieved he does not need to recite the berachah again, because he did not interrupt its recital with another matter (see Igrot Moshe, Orach Chaim, part 2, 22; Yabia Omer, part 6, 7).
One must be strict not to appoint a singer who is accustomed to singing indecent songs to be a regular chazan, or a chazan for the High Holy Days (Rama 53:25).
The poskim disagree whether or not it is permitted to use melodies of offensive songs for prayers and liturgy. In practice, when the congregation is not familiar with the vulgar song’s lyrics, it is customary to be lenient and adjust the melody to suit the prayer. But if the congregation recognizes the song, its tune may not be used for prayer, because when people sing it, they will be reminded of the crude theme of the song and their concentration is likely to be disrupted.