Rumblefish, the music licensing company that specializes in social media uses, today announced the release of its API to other developers. It also said users have licensed 4 million songs to date, and that its catalog has grown from 35,000 a year ago to more than 400,000 songs following a series of additional content partnerships,

Rumblefish founder and CEO Paul Anthony said the API makes it easy for third parties to integrate licensed music into their online products. It also can generate revenue for partners who share in music licensing revenues or who incorporate licensed music into premium versions of their application or website, for example.

Companies using the API before its public release include social movie-making app company HighlightCam and video editing service Clipik. Now that it’s out of beta, Rumblefish is accepting new partners from all industries including video, photo, gaming, presentation and mobile apps.

Both companies, in different ways, give users the ability to browse custom playlists, receive soundtrack recommendations by describing their videos, search for and filter music, and license songs as “social soundtracks” for any piece of user-generated content.

In 2010, Rumblefish launched the consumer product Friendly Music in partnership with YouTube, offering creators a simple, inexpensive and legal catalog they can use for their online video soundtracks. The API will extend similar capabilities to a wider range of uses.