This sound is a combination of the three letters- अ , उ and म् . The अ --sound signifies विष्णु , the उ -sound signifies शिव and the म् --sound signifies ब्रह्मा .

अकारो विष्णुरुद्दिष्ट उकारस्तु महेश्वरः मकारस्तु स्मृतो ब्रह्मा प्रणवस्तु त्रयात्मकः (vāyu purāṇa)

The sound " ओम् " is called " प्रणव " or " ब्रह्मन् ." All mantras begin with the sound ' ओम् '. Because of its sacredness, शूद्रस् and other low-caste people are not allowed to utter it or to hear it. They may pronounce it only as " औम् ." This sound includes all that has happened and all that is to happen. (Māṇḍūkyopaniṣad) .