"With this new feature, site owners can mark up their pages using the newly created music markup spec on schema.org, and search results for that site may start displaying song information in the snippet so that users know that there are songs or samples there for them to listen to. Several initial partners have implemented the music markup on their sites, including MySpace, Rhapsody and ReverbNation," explained Google.
The new rich snippets include links to music pages that could be found by visiting the search result, but it's faster to bypass the search result and start to play a song. It's important to mention that the songs aren't played on Google's results pages, so you still need to go to a different page. You'll only see the music rich snippets when your query includes the name of an artist, a song or an album.
Google also shows rich snippets for events, profile pages, recipes, videos, reviews and products. "With rich snippets, webmasters with sites containing structured content—such as review sites or business listings—can label their content to make it clear that each labeled piece of text represents a certain type of data: for example, a restaurant name, an address, or a rating."
At some point, Google will make structured data searchable and it will be a lot easier to refine results. Recipe search already lets you do that and it's pretty useful.