Not a UWP app as far as I can see, though it is available for Windows 10 on the desktop too, but Kauna is certainly unique, seemingly able to visualise your Windows Phone/Windows 10 Mobile audio directly from the OS, in a variety of pretty ways. There's also the option to use the phone's microphone if the music is coming from somewhere else. Is it any practical use? None at all. But it's definitely quite cool...!
Kauna is a music visualizer. Kauna renders any sound reproduction in system. It can also capture audio from a microphone. There is a choice of several types of visualizations: wave, confetti, bars, burning clouds, plasma and the sound level meter.
It's a surprisingly quick, fun and cool utility, even if it's not a UWP app under W10M and therefore doesn't support Continuum, since this would open up some uses cases, e.g. at events to provide interesting displays.
Here's Kauna in action:
The Plasma visualisation in action, along with a deliberately brought up set of selections and controls - this, amazingly, really does seem to work with either the microphone or the internal audio in Windows Phone and Windows 10 Mobile.
With a dB meter one of the visualisations, I guess this might be able to double as an audio meter in the field? You'd have to apply an offset or magnifier though (see 'Settings' below!) - it didn't seem accurate to me, as measured by an external device.
Everyone's favourite visualisation, the spectrum analyser (spectrum, get it?...) in action. The controls can be dismissed at any point for a clean look - just tap on the visualisation.
Only six visualisations are available at the moment, I'd imagine that the developer would love to get a few more on board.... ;(right) visualisation switching is done well, as is system and microphone sensitivity.