It is not permissible for the muezzin to omit any letters from the adhan, nor to add a letter during it, and likewise, he should neither omit nor add to the qualities of the letters: such as movements, pauses, elongations, and others; for the sake of improving the sound. However, merely improving the sound without changing the wording is good; for Yahya al-Bakaa, may Allah have mercy on him, reported that a man said to Ibn Umar: "I love you for the sake of Allah," to which Ibn Umar replied: "But I hate you for the sake of Allah." The man asked: "Why?" Ibn Umar said: "Because you embellish your adhan and take a fee for it." This is mentioned in Al-Mu'jam Al-Kabir 12: 264, and in Musannaf Abdul Razzaq 1: 481, referring to embellishment. As for exaggeration, there is no harm in it; because it is one of the dialects. See: Sharh Al-Wiqayah p. 140.