Why did the children stop in the doorway? Why did they close the door? What type of sound did they hear?

The children, who have no memory of sunshine, have, for the first time since arriving on Venus, played outside in the sun for an hour. It has been an extraordinary experience for them, joyful beyond what they could have imagined. Now, however, lightning, thunder, and rain are returning. The children stand in the doorway to escape the rain but hesitate going inside because they want to experience the last seconds of what the outdoors feels like before the rain takes over. They know these are their final seconds outdoors for seven long years. Bradbury puts it as follows:

They stood in the doorway of the underground for a moment until it was raining hard.

When they close the door, it means they are acknowledging that their glorious time in the sunlight is over:

Then they closed the door and heard the gigantic sound of the rain falling in tons and avalanches, everywhere and forever.

As the above quote indicates, what they hear is the rain falling in torrents, a sad and dismal noise that sounds imprisoning to them now that they have experienced what sunshine and the outdoors are like.