Omis more than a sound, but the vibration of the universe. It is the frequency of energy that connects and joins all things together. it is not a short and sharp sound Om. It lingers and flows like energy itself, Ommmmmmmmmmmm...
When you open yourself up to the vibration of OM you will feel yourself grounded not into the earth, but into the cosmos that surround you. Just like a raindrop dissolves into the ocean once it has touched the surface, once you feel the sound resonate in your throat you will feel the barriers that separate you from the ocean of Love that is all around you disappear.
To make OM, do not try to push air out through your vocal cords as if you were singing. Instead let the energy within pass by your vocal cords and resonate and vibrate as the air in your lungs flows past them. The first forces air out, the second simply allows air to flow from within your body to the world outside. That flow connects you to the energy within and to the energy without. In so doing Om connects you to everything that is.
It does not need to be loud or low. It does not need to be high or at a particular pitch. For everyone is different. It is up to you to find the shape of your own vocal cords and the vibration of your own energy, for that is where OM is found.