I acknowledge this graphic for Om from the Net.

Listen intently to the uncreated sound at the heart 

Like rushing waters uninterrupted

Reach the Supreme Reality as sound

This is a dharana Shiva prescribes for the anahata chakra, the heart energy center. The anahata chakra is generally associated with the elemental energy of air and the sense of touch. Here, Shiva links it to sound.

Anahata, literally means unstruck or uncreated. It represents the sound of creation, that which was not created.  In the earlier dharana of the shanmukhi mudra (or as some call it yoni mudra) as well, sound was one of the focal elements in addition to light. In this dharana it is just sound.

Several scholars, Swami Sivananda and others, refer to this sound emenating ftom the anahata as the nada brahman. They teach nada yoga, the yoga of sound based meditation upon the anahata.

Anahata represents the shift from an internal to an external fous in our energy. From the muladhara, the root center,  till the manipuraka, navel center, our energy is focused inwards. At the anahata, we start relating to the external world. We start looking at relationship, love, compassion and such other positive feelings when this center is energized. When it is blocked in energy we feel energy drained in these aspects desperately seeking attention from others.

If after meeting some people you feel drained of energy as if they are sucking it from you, they have a blocked anahata!

Sound can be experienced at different levels. We can say something aloud and listen to that sound. This is gross sound vaikahari. The sound can be whispered as madhyama. At a subtler level, we can mentally think about a mantra or sound and play it as it were in our mind space as pashyanti. This is more powerful and subtle. At a much subtler level the sound plays itself with no thoughts. You may call it transcendental sound or shabdabrahman.

This is perhaps what the Zen Koan of the sound of one hand clapping is about!

Some say that chanting Om is the gross level experiencing. Mentally reproducing Om in the mind is more powerful. The most powerful is when Om plays itself without your effort. This is the anahata sound Shiva refers to.

In deep meditation, bring your attention to the heart region, the anahata. I find it easier with my practice of yoga nidra, finally bring my awareness to the anahata, the seat of inner consciousness. Let go. Focus of the sound that you hear. With practice you will hear a sound. Eventually, this may be the pranava mantra, Om. 

In the Vedic tradition the pranava mantra Om is the origin of everything, similar to the Biblical saying, ‘in the beginning there was the Word and the Word was God.’ here, in the beginning there was the Sound , the Sound was Om, and Om is God.’

Shiva’s dharana leads us to that Pranava , Om, which is the supreme reality Brahman.