When I learnt English grammar I was told that some of the senses don't get "ing" of present continuous, therefore we should say "I see it now" rather than "I'm seeing it now". On the same logic, "I hear you now" rather than "I am hearing you now". But I doubt it in practice. I want to talk freely but at the same time to be sounded natural. When should people keep the mentioned rules in practice, if any?
For example: Imagine a teacher in a lecture who doesn't(?) hear his student while he ask him question. Should he tell him:
I am sorry, I don't hear you, so please, speak a little bit loudly.
I am sorry, I am not hearing you, so please, speak a little bit loudly.
Kindly, inform me if there should be a difference between formal and informal language in this case.