Not immature style - We already have another app for younger children, ages 3-7, based on the fully illustrated children’s book. This app is for older children and adults. Should not be immature and distracting, but also not boring.
Avoid visual distraction - Many users will have disabilities related to visual or cognitive distraction. Colors and 3D need to be subtle. This app is not for the visually impaired, merely those who struggle with dyslexia/dysgraphia. The strongest visual element needs to be the letters on the cards themselves. Other design elements should not distract from them.
Avoid words - What? No words? Well, minimize the use so that children, or adults who can’t read, or don’t know English, can understand how to use the game. Specifically the action button icons, play, listen, repeat, stop, home, need to be picture icons, rather than words. Some parts will be intended for teachers or parents to set up, like the settings page and more info page.
Differentiate from Montessorium Letters - Montessorium did a great job with their app. http://montessorium.com/letters/ It has a beautiful, simple, and elegant design. But it is a different teaching method and does not incorporate auditory or other important pedagogical elements. We need to differentiate from this app, even though we like it.