As I was digging through the latest build of Google Play Music, I noticed something strange: lots and lots of YouTube stuff. "That's odd," I thought, "What does YouTube have to do with Play Music?"
Oh, right, music videos!
Sure enough, there's some fairly revealing text included, too:
<string name="finding_videos_for_track">Finding related videos for the track...</string>
<string name="no_videos_for_track">No videos found for the track.</string>
<string name="youtube_video_details_hd">%1$s | %2$,d views | HD</string>
<string name="youtube_video_details_nonhd">%1$s | %2$,d views</string>
While listening to music, you'll be able to tell Play Music to hunt down the YouTube video for that song. It looks like the video will play in the music app, since there's lots of "video" views and styles in the xml.