Thank you for using Apple Support Communities. I see that you'd like to record audio over tracks in GarageBand on your iPhone. I'm happy to help. Take a look at the steps below:
Record sounds with Audio Recorder - GarageBand for iPhone
Record sounds with the Audio Recorder
Using the Audio Recorder, you can record your voice, an instrument, or any other sound using the microphone on your device, and play it back in GarageBand. You can also record sounds using an external microphone connected to your device.
The Audio Recorder has two sets of controls you can use to change the sound. Fun view lets you quickly change the sound of your recording, while Studio view gives you more options to enhance your recording.
Switch between Fun and Studio view
Record a sound
The following link also has additional information on how to use other features of GarageBand:
GarageBand for iPhone
Good luck with your challenge!