There are several concerts in Istanbul throughout the year. You may see an open air concert in Harbiye, listen to a classical music concert in Hagia Irene Church. Also Taksim is one of the most important location, where you may find many popular concerts.
This video shows the Carmina Burana by Carl Orff, performed by Istanbul State Symphony Orchestra in the impressive Church Hagia Irene. The church is from the 4th Century, and was commissioned by Constantine I. It is located in the outer courtyard of Topkapi Palace.