While practicing Hong Sau, the tongue or throat should not move, but i cannot mentally chant the mantra without slightly moving the the tongue/throat (very slight), especially at the G in Hong and S in Sau. I tried to listen to the breath also, but it does not make much sound (especially when shorter). I then tried imagining someone speak Hong Sau while i breath, but that is mentally tiring rather than relaxing. Please help.
You are right, the tongue and throat should not move. It is a very important aspect of the Hong Sau technique to become a “silent observer,” as Yogananda teaches. Try to observe the breath as if it were the breath of someone else. In this way you detach from the body, which is a very important spiritual aspect of the technique. The soul (your true Self) is the observer. During Hong Sau you get closer to that state of being a free observer. You change your sense of who you are: not the body, but the ever-observing soul.
These are Yogananda’s words: “By watching the breath, you metaphysically destroy the identification of the soul with the breath and the body. By watching the breath, you separate your ego from it, and know that your body exists only partially by breath.”
So really do watch, observe, without moving anything in the body.
You can train your mind outside of meditation to repeat mentally “Hong-Sau” with the breath, to get accustomed to it. It is not that difficult.
Listening to the breath (in the sense of hearing it), as you write, is not easy at all. The sounds “Hong Sau” are the subtle sounds of our astral breath (the currents flowing up and down the spine as we inhale and exhale), and can be heard only by highly advanced yogis. What we can all do, however, is imagine that the breath makes these mantric sounds.
Also imagining that someone else speaks “Hong Sau” as you practice, as you say, seems quite too difficult. I tried it and yes, it is tiring.
You will soon find out, I believe, that it is not difficult to watch the breath and accompany it inwardly with the mantric sounds Hong and Sau. Just try a little longer, and it will become natural for you.
Have you participated in guided meditations? It can help a lot. You can find a guided Hong Sau meditation here.
Finally, try to absorb this teaching of Swami Kriyananda (from his book Awaken to Superconsciousness), which might inspire you to watch, to become a silent observer, without moving anything in the body: “Hong-Sau will help you to convert ego-consciousness into the complete awareness of who and what you truly are: a manifestation of Pure Consciousness.”
All the best to you in finding out the way to successfully silently chant this ancient and sacred mantra in your meditations.