Once you've opened up a new audio post, the easiest thing to do is to just start typing an artist or song name. If it’s on Spotify or Soundcloud, we’ll find it and post it for you. You can also click on the little headphones icon on the right and post your own MP3s—one per day, 10MB max—and update the artist, title, and album info once the file has been processed. M4A, M4P, and WMA files don’t get along with our uploader, and we don’t accept URLs that point to a web page or Flash embed.
And in the mobile apps, tap the little audio icon (see those uppy-downy lines that look like a sound wave?), in any post type, to search Spotify or Soundcloud to add songs to your posts.
Note: For arcane reasons, you can only make audio posts by uploading a MP3 from the web.