Visitors to Orthodox Churches will always encounter a "sung" worship service. It seems singing is all the Orthdodox do in the Liturgies and services of the Church. There are, of course, exceptions. However, the primary service most visitors (and all too often even Orthodox Christians) attend is a Divine Liturgy.
Depending on where the Faithful of the visited Church come from --- Greece, the Middle East, or Eastern Europe --- most non-Orthodox visitors in America will either hear musical melodies and modes that are familiar to them; i.e., "Western" style with four-part harmony often referred to as Slavonic, or something that sounds very exotic and foreign or Middle Eastern. This "foreign" sounding music is most often termed simply as Byzantine.
The musical beauty of Orthodox Christian worship is, like the Church itself, a 2000-year-old history with which all Christians --- espeically Orthodox Christians --- should know and with which they should become familiar.
An excellent resource for initial study of the music and hymnology of the Orthodox Church can be found at Ligurgica.com, and you are encouraged to visit this wonderful resource where you can read about the liturgies of the Jewish, Early Christian, Orthodox, Western, and even Protestant churches (note the menu tabs at the left of this resource page).
It is not uncommon in many American Orthodox Churches, especially those of the Antiochian Orthodox Christian Archdiocese with its Middle Eastern but numerous American converts to Orthodox Christianity, and the Orthodox Church in America (OCA) with its Russian Orthodox roots, to hear a mixture of both Byzantine and Slavonic hymn, melodies, or tones.
This is how it is a the Parish of St Timothy in Fairfield, California. Fear not! Non-Orthodox visitors, and even Orthodox Christian visitors, will be able to understand all that is being sung and said as our services are in English -- the "common language" of America (and most of the world).