You can turn up the volume of your plugged-in Apple's EarPods or In-Ear Headphones in several ways, but they won't work correctly if your device or accessory is dirty. Before you turn up the volume, gently tap or brush off debris in the iPhone's speaker jack, the earpieces and between the headset's remote mic covers. Then, restart your iPhone.
Use Hardware Controls
Your iPhone and Apple headset each provide volume buttons that regulate the sound you hear over headphones. The iPhone's controls are on its upper-left edge, while the headset's controls are on the remote mic on its cable. Press either item's top button marked with a plus sign to increase headset volume.
Use Touch-Screen Controls
Music and media that you listen to or watch on the iPhone typically display a volume slider on the touch screen. Drag the slider to the right to get louder headset sound. When no volume controls appear -- such as with Visual Voicemail and game apps – use the physical volume buttons instead.
Increase Volume Limit
If the sound you hear over an Apple headset continues to be soft despite maximizing volume with buttons and sliders, the iPhone's volume limit may be set low. Tap "Settings," followed by "Music" and "Volume Limit." Drag the slider to the right to increase the volume limit incrementally. If you're prevented from making adjustments, then there's a restriction in place that you'll need to disable. Tap "Settings | General | Restrictions | Volume Limit | Allow Changes." Enter your Restrictions passcode when prompted.