MUVIZ Nav Bar Audio Visualizer is a free app, that's simple to use and allows you to see a visual representation of the audio files that are playing on your device.
The best thing about this app is that it adds features that are highly customizable. You'll be able to watch the visualization in the lower corner of your screen, or on top of the NavBar, or even completely personalize it any way you like in terms of both color and shape.
You'll also get the option to specify what kinds of apps you want to see in each bar, and where you don't want them to appear. Even so, this added feature disappears automatically whenever an app is in full-screen mode, so you won't be bothered in your upper or lower bar. It'll also hide whenever your keyboard opens, or if it's ever in conflict with another similar visualization system that uses the same app. For example, it'll hide if you're running Spotify.
