SoundCloud
With SoundCloud API you can build applications that will give more power to control your content
View Spotify music catalog, manage users' libraries, get recommendations and more.
Spotify API is a free Music API that requires OAuth authentication, is served over HTTPS and has unconfirmed CORS support.
Retrieve metadata from Spotify content or control playback
Details from beta.developer.spotify.com.

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With SoundCloud API you can build applications that will give more power to control your content
The Spotify API is provided by Spotify for Developers (beta.developer.spotify.com). Retrieve metadata from Spotify content or control playback
Yes. The Spotify API is listed in the Music category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Spotify API uses OAuth authentication, so you need to register for credentials before making requests.
CORS support for the Spotify API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Spotify API is served over HTTPS, so it's safe to use in production.