At the Pixel event in October, Google introduced a new always music listen offline recognition feature which would automatically detect the song playing nearby. At the same event, Google mentioned that general recognition feature to the assistant would be rolling to Non-pixel phones as well and today that feature is being rolled out.
Though the Google Voice search (Google Now) had this feature, for some reason, Assistant was lacking that till the Pixel 2 and Pixel 2 XL were launched. If you have your Google Assistant voice set as the “preferred input” method and are in a cafe or restaurant where a song is being played out loud, you will see a new “What’s this song?” chip as the first suggestion in the Assistant carousel if you trigger it. This chip doesn’t show up if no song is being played in the background.
Again, you can manually request Assistant to recognize “What song is this?” and it will start listening to the song. it displays YouTube, Google Play Music, Spotify shortcuts for the song. However, it looks like this feature isn’t live on all the supported phones yet, and functionality may be regionally limited.