Sheikh Yûsuf al-Qâsim
It is true that the one who makes the call to the prayer should raise his voice particularly in situations where loudspeakers are unavailable. He should enhance his voice and beautify it.
The Prophet (peace be upon him) told `Abd Allah b. Zayd to inform Bilâl of the words he heard in his dream so that Bilâl could make the call for prayers using those words, since he had a better voice. [Authentic hadîth related by Ibn Mâjah and Abu Dâwûd]
Ibn Hajr comments: "This means Bilâl had a better and higher voice that could reach more people."
Some people of knowledge dislike the modulation and elongation of vowels in the adhân if it affects the pronounciation, since this might change or obscure the meaning of what is being said.
The hadîth you have mentioned about Ibn `Umar is weak.
Nevertheless, long and excessive modulation and elongation of the vowels in the adhân is not something recommended.
And Allah knows best.
Source: Islam Today