Music All the Time on Your Nexus 7
Most of the music you have in your Google Play Music archive is actually stored on the Internet, not on the Nexus 7. This arrangement holds true even for music you've purchased at the Play Music Store; it's kept on the Internet, not the tablet's internal storage. As long as you have an Internet connection, your tablet can play the music in your library. But what about when you don't have an Internet connection?
To make your music available offline, you need to pin it to the tablet's storage. That sounds weird, but it's a virtual pin. Here's how it works:
View your music library: Display the sidebar in the Play Music app and choose My Library.
Locate the song, artist, or album you want to keep stored on the tablet.
Touch the Menu icon by the song, artist, or album.
Choose the Keep on Device command.
The Nexus 7 ensures that the music is transferred to the tablet from the Internet and therefore is available to play all the time.
The music you've chosen to keep on the device is downloaded from the Internet to the Nexus 7. It will be available always, even when you're not connected to the Internet.
To review the music already on the Nexus 7, go to the main Play Music app screen. Touch the More icon beneath the My Library heading and choose the On Device command.