Voddio, by VeriCorder, is the multi-track audio and video editing app I use.
It’s very similar to older cousins AudioPro and 1stVideo, but Voddio’s user interface is cleaner and easier.
The learning curve is fairly steep — the swipes, hidden menus, and ducking audio are not terribly intuitive.
Still, while I’ve tried quite several other audio and video editing apps, Voddio remains my clear choice.
Let me know if you have any specific questions.