I was recently in Marrakech and Istanbul, two cities where you can hear the Islamic call to worship.
In many countries, Adhan (in Arabic) or Ezan (in Turkish) is the call to prayer that is recited 5 times throughout each day in order to summon muslims for mandatory worship. This practice is intended to reach as many people as possible, which is why microphones and loudspeakers are utilized. The Muezzin is the person chosen for this task, based on his talent at reciting the Adhan beautifully, melodically and loudly. It is one of the most important duties in the mosque.
What are they saying? That there is no strength or power except from God. That there is no God but Allah. That Muhammad is the messenger of Allah.
The first time I heard it I was in Marrakech. I just stopped and stood still and listened. It was a truly beautiful thing. After a week in Marrakech and a week in Istanbul I can honestly say that I never tired of hearing it. In Istanbul, it sounded like a beautiful song.
My favorite part? How the different Muezzins at the different mosques seemed to be speaking to each other. You would hear it coming from one mosque, then suddenly realize it was coming from another, then another, then back to the first mosque.
Experience it for yourself.