It was heard in a 'waj' that a speaker said, "Muadhdhin of the Prophet hadhrat Bilal r.a. couldn't pronounce 'Ashhadu' properly. He used to read ش(sheen) as س(seen). People objected about his pronunciation, and he was dismissed from the duty of adhan and another person was fixed for adhan. After a day passed, Jibril a.s. came to the Messenger of Allah, and said, "was not the adhan pronounced in your mosque yesterday?" The Messenger of Allah, peace and blessings be upon him, said, "Yes, adhan was recited more beautifully." Jibril a.s. said, the previous adhan used to reach up to Arsh. But yesterday's adhan had not reached Arsh. "In the sight of Allah, the pronunciation of Belal's Sin is Sheen."

Listen, this is a fake and baseless story. It has no relation with the hadith of the Holy Prophet sallallahu alaihi wa sallam. It is known from the clear statements of Sahih Hadith that Bilal R. was very articulate. He had a high and  melodious voice. That is why he was nominated to give adhan. The commentators of the hadith asserted that there is no basis for the above mentioned narration.

Almasnu' fi Ma'rifatil Mawdu' 113; Almaqasidul Hasanah 1977; Kashful Khafa 1/411