How do i adjust the volume for my headphones on the itunes library as everytime i listen to music on phone via my headphones the left ear is a lot lower then the right?

I have changed my phone (iphone4s) thinking it was an issue with the headphones intially but then saw that it worked perfect in different handsets so thought it was an issue with the handset. When i went to apple they tried a pair of headphones on it and agreed the handset needed to be changed. When i received the new handset tried out youtube an everything was working fine. However, now i've put songs back on itunes it is happening again. I cant find any location in settings giving me the option to increase the volume.