Sound does not travel at the same speed even through different kinds of gasses. The lighter the gas, the faster the sound travels through it, if the pressure and the temperature of all the gasses are the same. If the air we breathe were hydrogen, instead of being a mixture of oxygen and nitrogen, we should hear a clap of thunder much more quickly than we do. On an average, sound takes about five seconds to travel 1.6 kilometers through air. If a storm is 6.4 kilometers away from us, we hear the thunder about twenty seconds after we see the lightening. If the air were composed of hydrogen, we should hear the thunder about five seconds after the flash.
In a thunderstorm it is sometimes fun to figure out how far away the lightening is. For every five seconds that you can count from the moment you see the lightening flash to the time you here the thunder, you must add 1.6 kilometers to the distance between the lightening and you.
No sound lasts forever as a sound. The waves that carry the sound become weaker and weaker, and finally our human ears can hear them no longer. Nor is there any scientific instrument, no matter how delicate, that can record sounds after a certain length of time has passed; because the waves that carried the sound to our ears have ceased being audible.