Use Soundcheck to adjust the recording level in the app prior to starting a recording. To do this, tap the “+” sign to create a new project, then tap on the large image of the mic in the center of the recording screen to bring up the sound meter and slider adjustment. While a test sound source is active, adjust the slider so the colored level indicator never enters the yellow or red zones.
During recording, you'll see the .wav sound form moving across the app screen. If you see the lines of the .wav form turn red, consider reducing the volume of the sound source, moving the mic further away from the sound source, or adjust the sound level again before starting a new recording.
See video below:
More info on getting good sound can be found here.
Note: At this time sound level cannot be adjusted during recording.