The thermosphere is a layer high in the atmosphere known for extremely high temperatures and extremely low density. From about 50 miles above the surface of Earth upwards to 300-600 miles, what few air molecules exist are bombarded by high-energy solar radiation that keeps these molecules highly agitated and so, by definition, hot. Temperatures range between 1000 and 4500 degrees Fahrenheit, depending on solar activity. However, this air would not feel warm to us because it is so thin as to be almost a vacuum.
Throughout most of the thermosphere, sound waves cannot be transmitted because of the lack of a medium. With air density so low, there cannot be sound. It is within the thermosphere that the aurora borealis sometimes glows. The thermosphere is the outermost layer of the atmosphere. Beyond it, there is space.