Windows Universal app is being made as a part of the foobar2000 mobile project. We're hoping to make it available to general public next year.
<rant mode>
However, the development is very slow due to very poor quality of Microsoft's development tools and bad documentation. Our iOS and Android ports are functional already and neither of those platforms has given me as much ******** as Windows Phone / Windows Universal.
Honestly, having developed for Windows using Microsoft Visual Studio since 1998, I'd rather write Mac software than deal with this nonsense, Apple tools generally work and the documentation makes sense. I'll be glad to resume the unfinished OSX port when this is over and you have Microsoft to thank for that.
<end rant>
It is
not easy to convert classic Win32 to Universal Windows Platform (UWP).
UWP is has more in common with Windows 8 "Metro" and Windows Phone than with classic Win32.
You need to redo significant portions of Win32 API calls ( though many of old functions seem to have been dropped just to piss people off and get them using wrappers ), rewrite all user interface and use entirely new filesystem API to access user's music and documents libraries.
Background music playback must be done using a separate process ( implemented in fb2k mobile ) and run through a new set of APIs,
which are buggy.
foobar2000 for UWP is making progress, since the codebase and UI is 100% shared with the Windows Phone port, but you're better off using classic foobar2000 for Windows for quite a while, at least until Microsoft fixes their bugs.