Everything seems to be working fine. You’ve passed all the tests but I sort of feel like you did the bare minimum and thus you haven’t challenged yourself as much as you could. At the least I would think that any app that produces sound would have a volume control. Adding at least another sound bank would be cool.
There is one accessibility issue that should be fixed though. The black text on dark gray background doesn’t have enough contrast to be easily readable. You’ll need to lighten up the button color. Use the WebAIM color contrast checker to make sure it is accessible.