Pronunciation of ॐ, OM 唵、嗡 or AUM

How the Sanskrit sign ॐ should be pronounced? 

ॐ is used frequently in meditation or in dharani, which is a sign of combination of three Sanskrit letters, आ A, उ U, and म M. Therefore, by nature, it pronounces as A-U-M. However, it’s been translated into Chinese character as 唵 or 嗡，and pronounces as ‘Om’. Until recent years, some teaching videos on the internet show that the sign ॐ pronounced as ‘A-U-M’ instead of ‘OM‘. Even some yogis teach the sign be pronounced as  ‘A-U-M’ not  ‘OM‘. 

I’m glad to find the comment made by Sautrik Bhattacharya on YouTube shown as below, which I think has the most reasonable explanation. It makes ‘grammatical’ sense at least!

“…you are right. They are three sounds a, u, m but when they are pronounced together then a+u becomes o according to the rules of sandhi of Sanskrit grammar which deals with the mixing of different sounds. It comes under guna svara sandhi. Like hita + upahaar = hitopahaar, gyan + upahaar = gyanopahaar. The rules are based on universal law of flow of words. So Om is the correct pronunciation.”