Muslims use the human voice to announce the times of prayer to the community. The adhan is the call to prayer itself, and the person who calls it is called the muadhan.
The following is the Arabic transliteration and the English translation of what you hear:

Allahu Akbar
God is Great
(said four times)

Ashhadu an la ilaha illa Allah
I bear witness that there is no god except the One God.
(said two times) ...more

Source: http://islam.about.com/cs/prayer/f/adhan_english.htm

Allahu akbar, allahu akbar, allah hu akbar allhu akbar'
Ashhandan la hi la ha illal-la, Ashhandan la hi la ha illal-la
Ashadanna Mohommadaur rasool Allah, Ashadanna Mohommadaur rasool Allah
Hainya las salath, haiyan las salath, Haiyan Lal falah, Haiyan lal falah
Allahu akbar, allahu akbar, allah hu akbar allhu akbar', la hi la ha illal la
The Adhan at the time of prayer included Qadqmatis salath, Qadqamatis salath