OM

Om

a, has been mentioned in so many scriptures, from Patanjali Yoga Sutras, to Bhagavad Gita, and Upanishads.

Lord Krishna says this repeatedly in Bhagavad Gita In the 7th chapter, He says

 aham praNavaH sarva vEdEshu

 I am praNava, among all knowledge. In the 9th chapter, he repeats,

 aham vEdyam pavitram OmkAraH

 I am the pure sound OM, that is to be realized. In 10th chapter he says it again,

giRAmasmyEkamaksharam

 Among sounds, I am the single understroyable sound, which is Omkaara.

Om symbol break down

In His Yoga Sutras, Patanjali says

 tasya vAchakaH praNavaH

(27), meaning the sound that denotes God, or Ishvara is praNava. Continuous meditation on the sound, and meaning OM, one of the ways of doing Ishvara pranidhAna, is seen as an effective way of achieving the objective of Yoga. Om is the divine, Om is everything that exists.

mAndukya upanishad says :

 Om-ity-etad aksharam-idam sarvam 

Om is the sound which is, everything that is manifest.

This same knowledge is expressed in various ways in chandogya upanishad, brihadaranyaka upanishad, svetAsvatara upanishad and many many others.

Infact, atharvashikhA upanishad describes each individual sound inside Om in so much detail, that it needs to be covered in a video of it's own!

Many cultures have tried to express this in their own terminology. They had their own ways, laid down by masters, who mastered this capability of accessing divinity within.

This same sound Om, is what is being chanted as Ameen in Islam, and Amen in Christianity.

So, if there are many version of this sound, what makes the Sanskrit way of pronouncing it right ? It is because of the significance of what happened in the land protected between the Himalayas, and the Indian Ocean. It is the number of masters this protected culture produced.

We had so many people who became fully capable of accessing the divinity within, We crafted language, arts, dance, music, math and medicine, all of it focused on one thing. Enhancing the human capability of turning inward and accessing divinity within.

We even called our land bhAratam, and ourselves
bhAratIyAH, the land that revels in light of existence, and the people who are in tune with it. That is exactly why sanskrit holds such a treasure of invaluable information.

Om Explained - The definition of Om - AUM - Ohm - Omm Sound and Symbols.

Sadhguru Share The Secret Behind The AUM Chanting | Science of Sound | Mystics of India

You must know:

the human system is capable of other sounds are only a mixture of OM.

you just experiment with yourself if you remove this tongue😝 sorry, you just keep it still not using it, you will see the only three sounds that you can utter “aaa , uuu and mmm” the only three sounds you can make without the tongue. 
Tongue is being used to mix these three sounds and produce all the manifestations of sounds mixing them in permutations and combinations you can produce millions of sounds, so these three sounds are known as the basic sounds are the fundamental sounds in the existence.