Soundcloud oneboxes work great with Discourse. For Spotify, if you add a Spotify link to a song or playlist to a Discourse post, it creates a onebox, but not a Spotify player.
You can allow users to add the Spotify embed code (iframe) to Discourse posts by adding https://open.spotify.com/embed/ to your site’s allowed iframes site setting. This will create an audio player that lets users listen to 30 second snippets of tracks.