Some people hear the soundSaham when they listen to their breath instead of Soham. This is perfectly acceptable and can be used in shabda sanchalana instead of Soham.
Soham should be integrated with the breathing. With the practice of ujjayi pranayama the sound can be heard very distinctly. Try it. You should hear: So with inhalation, Ham with exhalation.
In shabda sanchalana you should hear so as you breathe in from mooladhara up the frontal arohan psychic passage to bindu. The sound should be prolonged.
That is, from mooladhara to swadhisthana kshetram the emphasis will be on the sound: s-s-s-s. From swadhisthana to bindu the emphasis should be on the sound: o-o-o-o. This is clearly shown in the diagram.
You should hear the sound Ham as you breathe out and down the spinal awarohan psychic passage from bindu to mooladhara. From bindu through ajna to vishuddhi, the emphasis will be on the sound: h-a-a-a. From vishuddhi to mooladhara the emphasis should be on the sound: m-m-m. This will be a humming sound that vibrates down the spine.
The sound Soham should not be pronounced aloud. It is the natural sound of the breath. The sound should be such that you can hear it very distinctly, but other people cannot unless they put their ear very close to you.
Remain in the same sitting pose that was utilized for pawan sanchalana, the previous kriya2. Only move and adjust your position if you feel any pain and stiffness in your body.
The traditional sitting poses are padmasana or siddhasana (siddha yoni asana for women). If these asanas are not comfortable then you can use any other comfortable sitting pose such as ardha padmasana, swastikasana, etc.
Keep your back straight throughout the entire practice; the only part of the body that should be moved is the head.
Psychic passage awareness
As you breathe in with So your awareness should ascend the frontal arohan passage. After leaving mooladhara chakra you should feel the mantra and breath piercing swadhisthana, manipura, anahata and vishuddhi kshetrams and then finally bindu.
As you breathe out with Ham your awareness should descend the spinal awarohan passage. After leaving bindu, feel the mantra and breath progressively piercing ajna, vishuddhi, anahata, manipura, swadhisthana and finally, the mooladhara chakra.
There should be no mental repetition of the names of the kshetram and chakras . . . only awareness.