Sound is a form of energy of course,it can propagate through air.The reason we hear each other when we speak even from a distance apart.However,it's important to know that sound behave like a vibration or it can be said to be a vibration in it own.Hence when sound does not hit an obstacle or a body ,to return,incase of Echo.It gradually diffuse without any effect.I don't think it turns to another form of energy.

Suffice to say that i stand to be corrected on this.I have only answered this base on my knowledge.

To explain further on this topic,you can notice how vibrations make sound by plucking a guitar string.The string vibrates back and forth.The vibrating string makes the air around it to vibrate.The vibrations make sound wave in the air.The sound waves travel through the air to your ears.Thereby making your eardrum and the interior of your ears vibrate.Your ears then send a signal to your brain.Finally, your brain tells you or manipulates the information that you are hearing a guitar string.

Sound wave is not visible unlike water waves that you notice when you drop a pebble in still pond or a big tub of water.However,sound could be said to be low or high-that is sound of high frequency and that of low frequency.

With the information on this article you will be able to know how sound behaves and how it is propagated through a medium,be it water or air.Most importantly,i think sound could be said to decay after you must have heard it,rather than turn another form of energy.