Sound issues on iPhone 6S?

Its obviously not the iPhone. Since this is the newest iPhone the processor is faster than my iPad, which the sound works fine on. The sound started fine, actually better than my iPad, and it ran much faster on my iPhone than my iPad since the processor is much better and faster. Then out of nowhere everything started crackling loudly, and it played at about half tempo. There was one piano playing. So I reset my phone. Still there. I erased the app, reset my phone, download it. Still there. What?

WNN8 about Notion