Amritabindu & Amritanada Upanishads

The Amritabindu Upanishad and Amritanada Upanishad are said to have been composed between 100 BCE and 300 CE, and influenced by the tantric philosophy of the time. Both emphasise the importance of the mantra Om as a vehicle to liberation. They are two of the thirty-two Kṛṣṇa Yajurveda Upaṇiṣads. 

Amrita-bindu has twenty-two verses. Amrita literally means deathless or immortal, representing the transcendent Self which never dies. Bindu is the seed containing the inherent source or power of sound and consciousness. There are two kinds of mind, pure and impure; it is difficult to liberate the mind from the impure, as it is attached to the experiences of the five senses. The craving mind is in bondage; the mind that does not crave is liberated. The repetition of om with sound can lead to the echo of om without sound. By meditating on the soundless, the yogin reaches the state of inner peace, the Supreme Spirit.

Amṛtanāda Upaniṣad has thirty-nine verses. Nada in the form of sound is the first vibration from which all creation has emerged, being the first manifestation of the unmanifested absolute. The mantra om is the vehicle the yogin uses to reach Brahman. Once the yogin has attained Brahman, he has no need of the mantra. Willpower can be strengthened through the sixfold path of yoga, retention of the breath and gayatri mantra. Nadi shodana, alternate nostril breathing, should be practised to prepare for meditation.