Why we can't hear the tundher's sound in the night?

Answer 1

Thunder is sound. And it usually happens at night. So I don't understand your question.

Answer 2

What you are referring to is a simple trick between light and sound and the night sky. The night just makes it easy to see--pardon the pun.
You CAN hear thunder at night, especially if it is close to you. What is confusing you is that sometimes you can see the lightning but you do not hear the thunder. That is because the dark night allows the light from lightning to be seen from much much farther away. The thunder is still there, but by the time it arrives, it is so faint, that your ears may not be able to detect it. This is especially true if there are other things around making noise.
If you could see a flash of lighting in the far distance at night, and not hear the thunder, it is most likely that you would not hear the thunder during the day as well. On the other hand, you would not be able to see the lightning flash that far away during the day.
This is similar to why you can not see stars during the daytime. The stars are still there, but their light is so faint that your eyes can not detect them because they are overpowered by the much more brightness of the sun.
So to answer your question, you are able to see lightning much farther away at night, and those lightning flashes are so far away that the thunder is to faint too hear at such a distance.