Harmony

Harmony is a modern, Electron-based piece of software that can play your locally stored music files and all your music from Soundcloud, Google Play Music and Spotify.

Easy installation and configuration processes

The application can be deployed on your computer in just a matter of seconds, since it comes with a streamlined installer that once started takes care of all the process without your intervention.
This said, subsequent to its effortless installation process, the application is automatically launched.
Before you get to lay your eyes on its actual interface for the first time, you are met by a compact window that prompts you to log in using your official Soundcloud, Spotify, and Google credentials. Please note that it is from this window that you can also choose the location of your local tracks.

Functional and user-friendly interface           

The interface is not what you call stylish but it does look right at home on modern iterations of Windows. The layout is quite straightforward with a top toolbar that allows you to play, pause, skip tracks, a typical music progress bar and a search field.
On the left side of the main menu, you can access all your music from the same place, as well as discover new music using the Feed feature. Both local playlists and the ones from the aforementioned music streaming services are available, as well as your the list of favorites.
You can activate the shuffle function from the bottom-toolbar and access the Settings section that allows you to choose a modern way to view your music library if you enable the Coverflow feature.

Packs more under-the-hood, little features that you would have initially thought

Despite its main advantage, that of bringing your local tracks and other cloud-based music sources all in one place, the app may seem a bit simplistic.
This said, features like support for your keyboard's media keys and keyboard shortcuts, a very reactive and smoothly resizable main window, system notifications and support for Last.fm scrobbling may convince you that this is actually a very well-built app.

Modern, versatile and efficient, Electron-based music player

All in all, if you want a user-friendly music player that can bring together your Spotify, Soundcloud and Google Play Music and your local audio content, all in one one place, and it works on all major platforms such as Windows, macOS and Linux, then Harmony clearly fits the bill and you should at least give it a quick try out to see what's what.