My answer is probably too late to be of any use to you, but I'll answer it for the sake of other readers.

You can use the solution in this tutorial to play long audio files as well as long as you store the files in you app assets. However, usually if you have a lot of long audio files, you don't want to make your app download size too big by including them with the app. It's easier to download them or stream them later. Also, for longer audio, you want to be able to pause and seek. That's not necessary with a clip which is only a few seconds long. So that's why this article didn't include that functionality.