Have the instructions be given completely before the kids are able to begin the skill.
For example, when my 2 year old sees the fruit skills he will automatically start tapping on the fruit without having heard exactly what he is supposed to be doing for that skill ie picking out the fruit that is different or matching all the fruit of the same color.
I think that if the instructions are given before the kids are able to start interacting with the skill then it'll help them to better understand exactly what they are supposed to be looking for.
Andrea Oddo commented
It's a great idea in theory and this really needs to be refined just a bit. My son is 4 and autistic but he is very very handy with computers/ipads/iphones etc. He is non-verbal and this game is extremely important to helping him with verbal proficiency (think 4 year old mind with 18 month old verbal skills). If you could make the "rules" of the game or "instructions" more self evident - like doing an example first screen where they can ONLY move the fruit (for example) across the screen to each of the right color categories (please keep it fun though!). There has to be a way to not give "adult instruction" and pulling them out of the game world. The instructions can be involving and intuitive as to what needs to be done without having to give invasive verbal instructions. I'm a computer professional with 20 years in the industry and know that this is the true value of tablets. They are much more accessible and personal - you are in that world and it is in your lap.
any audio input is good, great idea