My ds started doing the ABC reversing sometime between 2 and 2 1/2. He would sign and sing the letters at the same time. He probably did it for a year before he got bored with it. I don't know any apps as he is now 13.
I do remember he had a toy that would pronounced the words if you spelled a word correctly or just the letter sounds if it was a nonsense word. He could sit all day punching in letters to figure out new words. It was like a game or a challenge to him to figure out as many words as possible. He learn a huge number of words and could decode any word after a few months with that toy.
Maybe there is a similar app.
