MacBook Handsfree Problem

I have purchased new MacBook pro Retina Display 13 inch a month ago. My MacBook does not produce proper sounds when Connected to handsfree(iPhone 6/iPhone 4S).When i plug in my handsfree it gives low voice on the left side of the handsfree whereas it gives me full sound on the right side of the handsfree. I have tried both handsfree iPhone 4S and of iPhone 6. What I should do now?