Since the day I started using iphone 4 I always complained about the music volume while playing any song, as there is great difference in iphone sound volume while playing music when compared to ipod or ipod touch, the sound level on iphone is very low as compared to these devices.
To solve this issue, I came to know about a cool app called Volume Booster which is available on cydia under mod my repository. But in Order to install and activate it, you will need to install another app called WinterBoard if not already installed.
Note: You can easily find both of the above mentioned apps through cydia just by searching with their name.
In order boost iphone volume, first install WinterBoard and then Volume Booster and then launch WinterBoard app on your iphone and follow the instructions below.
1. After launching winterboard, tap Select Themes as shown in the image below.
Under select themes, select Volume Booster to activate it.
Once you activate it you will notice a volume boost in my case it was not a such a great difference but still it was louder both on headphones and speaker.