You can adjust the touch sensitity of the keyboard: Then the volume will depend on how hard you touch the screen: From the GarageBand help: (http://help.apple.com/garageband/ipad/1.2/index.html#chs39282dbe)
Increasing the touch sensitivity makes notes sound louder or softer depending on how hard you tap the keys.
- Tap the Keyboard button , and turn Velocity on.
- Pinch the two ends of the Velocity bar apart to increase the sensitivity range, or pinch them together to decrease the range.
- Drag the Velocity bar up or down to move the sensitivity range. This is particularly useful after pinching the ends closer together to limit sensitivity to a narrow range.
The same goes for the drums. If you do not want touch sensitivity, turn "Velocity" off.