It only has one speaker.
If it is only affecting the Music app, then check your setting for the Music in the Settings app. There is a volume limiting option.
If it is only affecting your system sounds (keyboard clicks, beeps, alerts, alarms, etc) Check Settings > General > Sounds. It is possible to set the level there and make it independent of the volume rockers.
Also, the headphone and internal speaker have separate volume settings, so make sure you have turned the volume up for the internal speaker, not just depended on a maximum headphone volume to be loud.
Otherwise I have little to offer, other than restarting the iPad and seeing if that helps.
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears.