Override Phone Volume and Use Radio Volume?

I recently purchased a 2011 Santa Fe. I was able to pair my iphone 5 with no trouble and I can even play music off the internet (youtube, Vevo) by pressing the "Aux" button. However, I noticed the radio volume controls are overridden and the volume is dependent on the volume settings of the phone, however, the phone is no where near as loud as the radio would be. Is there anyway to get the iphone to use the radio settings when I am playing music off the internet using the phone?

I don't have a USB of music files. I just click on either one of the above mentioned apps when I feel like hearing a particular song.