Sound meditation is very simple. You just listen to the sound and focus on it, staying with your mind on the object of meditation which is the sound. 

The mind tends to categorize sounds into sounds that are perceived to be pleasant and sounds that are unpleasant. But sound is simply made up of frequencies, and vibrations in the air. When we observe the sounds as they are, without judgment, we will find peace with what is. 

There are various types of sounds and thus various sound meditations.  On this page, you can listen to the peaceful sounds of the waterfall, judged by most people to be a generally pleasant and calming sound.