How to Adjust the Volume on Your iPhone 4S
Even though individual iPhone 4S apps, such as Music and Video, have their own volume settings, you can set your iPhone 4S system volume as well to help you better hear what’s going on. Volume settings in apps are based on a percentage of the system volume setting.
Tap the Settings icon on the Home screen.
In the Sounds pane that appears, tap and drag the slider to the right to increase the volume, or to the left to lower it.
Tap the Home button to close Settings.
In the Sounds pane, you can turn on or off the sounds that iPhone makes when certain events occur (such as receiving new mail or Calendar alerts). These sounds are turned on by default. Conversely, if you need the audio clue and this feature has been turned off, just tap the On/Off button for the item to turn it back on.
Even those of us with perfect hearing sometimes have trouble hearing a phone ring, especially in busy public places. Consider using the Vibrate setting in the Sounds pane to have your phone vibrate when a call is coming in.