Just about a month ago, the "what's this song" music identification functionality started to roll out to Assistant on both of the new Pixel 2 phones. Google's voice search (for instance, via "Google Now") had this functionality for a long time, but for whatever reason, the transition to the new Assistant stripped that feature away. Although it was back for the Pixel 2, it appears that music recognition in the Assistant is now rolling out to non-Google devices as well.
Just in case you aren't familiar with the concept, Google voice search—and, now, Google Assistant—were able to recognize music on the fly like Shazam or SoundHound. So if you were at a shindig or a cafe and some music you dug came on, you could easily find out what it was. Unfortunately, Assistant lacked that functionality when it landed, but it's an omission that's quickly being rectified </s>.
Music identification via Assistant on the OnePlus 5
We tried things out for ourselves, and although it hasn't started to appear on all our devices, we do see it on some. For example, the images above are from Artem's OnePlus 5. I tested it for myself on both my desktop-bound OnePlus One and the Nextbit Robin I've switched back to because reasons, and it's present on both of those devices as well.
The 'What's this song?' chip in the Assistant on both a Nextbit Robin (left) and OnePlus One on LineageOS (right)
Some devices we tested, like Artem's Galaxy S8+, aren't showing the feature just yet, though. We reached out to someone at Google on this subject, and we were told that it started rolling out to devices beyond the Pixel 2 last Friday.
The "What's this song?" chip should appear automatically if your device notices music playing in the background when you trigger Google's Assistant, though you can also manually ask it to identify music. The functionality may be regionally limited, though, as it was when it was a part of Google Search, so YMMV.
We reached out to Google and were able to confirm that the Song Search functionality is available in multiple regions.
Sound Search should appear in all countries that have the Assistant set up in one of the four eligible languages (en, fr, de or jp)
So if you have Google Assistant, and you're using either English, French, German, or Japanese, you should be able to use it, so it doesn't appear to suffer the same regional restrictions that it did previously.