mixing track with Hokusai

Hi I don't know if my answer will be helpful. When you are at "project" view you should be able swipe to the "add new track" at the bottom. There are 5 choices, the last is Dropbox. When I choose Dropbox it opened right up to the Dropbox account I am logged into in my Dropbox app on my phone. Then I was able to move through to the file I wanted to add. By the way, when you swipe through and get to "add new track", you don't have to double tap to select that, just keep swiping and it moves through the options to add new track. You could add various tracks to that project in this manner. Hope I didn't miss understand your question! Good luck

I haven't used the app that much but if you add more than one track to the project, they are layered and you hear them both when you hit play.

By double tapping the title (project name), you get several options such as "mix down tracks."

In case you didn't hear this podcast, this is a great walk through with VoiceOver...
http://www.applevis.com/sites/default/files/podcasts/applevispodcast36.mp3

Before someone can offer a step-by-step walkthrough, it may help if you're more specific about what you mean by "mix." Do you simply want to know how to combine multiple tracks into a single mono or stereo track? Do you want to know how to adjust the relative volume of these multiple tracks, or their position in time? I'm having trouble understanding exactly what you're trying to do, but if I had a more clear idea, I might be able to help.