Really like the concept, though it would be nice with some for options for the training mode as turning off the automatic answer or being able to increase the time before it shows. Another idea would be an option to remove the lines, as you after a while (and often in reality) only have your ears to help you.
Other than that - would love to see more kinds of gameplay like comparison of two intervals (e.g. which one is the highest?) etc.
 "Music Intervals"){kind=icon}
 "Frontier"){kind=icon}