All perfect praise to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
It is recommended that the Muezzin (One who calls to prayer) delivers the Adhan in a sound voice that observes the rules of recitation so that each letter is articulated as it should be in order not to change the meaning of the words and consequently invalidate the Adhan. This is because the purpose of Adhan is to call people to prayer. Al-Imam Al-Shafie (May Allah be pleased with him) said: "I like reciting the Adhan clearly and slowly without stretching its words, or singing them."{Al-Umm, vol.1 pp.107}.
Substituting a letter with another is a grammatical mistake that renders the Adhan invalid. Ibn Hajar said: "The Muezzin should avoid mistakes that invalidate the Adhan. Moreover, it is prohibited to pronounce its words in a manner that changes their meaning." {Reported from E`anat At-Talibeen, vol.1 pp.276}.
In addition, it is disliked to stretch the Adhan, as stated by al-Imam Ash-Shirbeeni in {Moghni Al-Mohtajj, vol.2 pp.170}.
In conclusion, Adhan should be performed in line with the rules of recitation without stretching, whether the period of delivering it was long or short, and we advise the Muezzins to perform it perfectly and avoid matters that could render it invalid. And Allah knows best.