Need an app to develop musical hearing

Hello! Can anyone recommend me an VO-accessible app aimed at developing musical hearing? I want to distinguish various intervals by ear, to learn the basics of cords and gammas (like how many signs in this or that tone), and such. I'm ready to pay a decent amount for the iPhone app. PS: If the topic has already been discussed, please, give a hint as to where to look for the info. Thanks to all in advance!