It is a famous fabrication mentioned in virtually every book on Mawdu’ ahadith.

In fact, Sayyiduna Bilal (RA) had no difficulty at all, rather he was very eloquent (fasih) and called the adhan loud and clear, which is why he was selected by the Noble Prophet ﷺ  for the task.
Ibn Kathir, Mulla ‘Ali Al-Qari, Al-Shawkani amongst others have declared this narration to be a fabrication. See: Al Maqasidul Hasanah, hadith: 582 and 221, Tazkiratul Mawdu’at pg. 101, Al Masnu’, hadith: 159 and Kashful Khafa vol. 1 pg. 411

These narrations have no basis and are fabrications. They are not suitable to be quoted. In fact the opposite has been reported, that Sayyiduna Bilal (radiyallahu ‘anhu) was eloquent and had a beautiful voice. (See: Al Maqasidul Hasanah, Hadith: 582 and 221, Tazkiratul Mawdu’at, pg. 101, Al Masnu’, Hadith: 159 and Kashful Khafa, Hadith: 695)

 And Allah Ta’ala Knows best

https://hadithanswers.com/the-seen-%d8%b3-of-sayyiduna-bilal-radiyallahu-anhu-is-a-sheen-%d8%b4-in-the-sight-of-allah-taala/
https://mawdu.wordpress.com/2012/12/07/bilal-athan-azan-seen-sheen-pronouncing-authenticity-source/