How to make iPhone 4S louder?

Me and my friend have the exact same iPhone except his is white. He. Never jailbroken it or anything but his is way louder than mine. How can I make mine loud like his WITHOUT jailbreaking it. Also his speaker looks much more cleaner while mine looks like it has a little dust on it.

Added (1). I know how to use the volume buttons I mean when both of our iPhones are max volume his still sounds way louder than mine

Added (2). I already set volume limit to off

Okay listen up! There's 4 buttons on an iphone! The first one is on the top. That's what they call the power button which turns on and off the phone! The second one is the home button at the bottom of the phone. This button actually lets you click out of the app that your running! Amazing! Then there's the last two buttons! They are located on the side of the phone! Now listen carefully! The one on top increases the volume and the one at the bottom actually decreases the volume! Just rememberwhen you turn your phone upside down Iits the other way!
Your welcome!

I think you have a volume limit on yours. You just need to change it.

Go to settings. Then go to Music. Go to volume limit. You can adjust it here.