Why don't we hear all the processes in our body (like the rushing of our bloodcells through our arteries, the pumping of our heart, the electricity of neurotransmitters or our brain cells, etc.)
Is it only because the sound is too low for us to hear?
The issue here is with the definition of the word "hear". We do hear most of this, that is our ears actually pick up and transmit this sound. But we don't hear it in the sense that our brain ignores it and filters it out of what we are aware of. Only a very small fraction of all the sensory data that comes into the brain ever gets processed. The brain does something called sensory modulation it filters through all the sensory data and decides what is worth paying attention too.
This happens at several levels, not surprising since we see these same levels of filtering in animals, as soon as you evolve sensory organs the ability to filter sensory information is an advantage. Some happens directly at the sensory organ, many have a threshold before a signal is sent. In the brain the reticular formation is responsible for much of it. But each sensory processing area also does some of its own filtering. Basically each portion of the brain contributes in some way. Attention and how it is controlled is greatly studied as it is central to how consciousness functions. A complete rundown of all sensory processing is tricky since some senses (like smell) skip some parts of the process, and there are so many ways it can be filtered. you can find a more detailed breakdown here.
The sound is mostly drowned out by all the other sounds in your environment. The environment is just too noisy. However you can hear it if you are place in sensory deprivation chamber. You can hear stuff like your heart beat, the eyelids moving, joints creaking, muscles tensing and even the movement of blood in the arteries within your ear.
You cannot hear neurotransmitter diffusion because it is a movement of chemicals, which do not generate a pressure wave and is thus soundless.
But there are sounds that you hear. You can hear yourself breathing and chewing your food.
