Interesting -- I didn't realize that Voice Search could automatically detect music like that.

(It also works with the microphone icon in Google Now.) 
What’s that song? Let Google help you find out! In December, we made the Sound Search widget for Google Play available in the Play store (http://goo.gl/M3a44). And here's a cool tip for music fans who are using the latest version of the Google Search app for Android:

You can now tap the microphone button in the search box on your homescreen. Within moments you'll see a music button appear telling you that Google hears the music too. Just tap the button to launch a music search and discover the name of the song and artist, and instantly buy the track on Google Play. 

Sound Search is currently available to users in the United States with the latest version of the Google Search app, for devices running Android 4.1, Jelly Bean, and above. 
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