How do I change the microphone input sensitivity on iPad 2?
What you can do with the built-in microphone is limited. You should investigate purchasing a third-party microphone, or better yet an audio interface with a microphone preamplifier, into which you plug a microphone. This will cost money but give you much more control.

Look for products from Tascam, Apogee, Line 6, IK Multimedia, and Griffin.