PB816

Hoping someone can help. Bought S4 at Christmas last year at Bestbuy and have been very happy with them up until the past week. All of a sudden, when listening to my songs, the music is okay, (not as powerful as it has been), but the voices are very, very weak. Any ideas????? Please help???? The bad thing is I have been searching high and low for my receipt but I cannot find it as of this writing. Thanks, PB