What the Node.js library for Beats Music does for a developer is to authenticate into an application or framework that supports Connect-style middleware. It is named Passport-Beatsmusic by Mark Purugganan. This passport is a strategy to authenticate using OAuth 2.0 API. Resources of this project include line of code for installation and instructions to configure and authenticate requests.
The Independent Online Distribution Alliance (IODA) has just announced its IODA Open API, currently in beta. The API offers many of the typical resources that you would expect from a music metadata service, with a few extra goodies like these album attributes: UPC code and primary label. Comparing album API results between IODA Open and iTunes, IODA is packing a lot more information into the response.
If you are looking for sound samples on the web and want to add them programmatically to your apps, the Freesound API may be just what you need. Freesound is a collaborative database of Creative Commons Licensed sounds. Users can join Freesound and upload their sound files and share it with others. Recently it also announced that it has crossed 100,000 songs in its database.