Microsoft is not quite done with Xbox Music for Windows Phone 8.1. Tonight, version 2.6.657.0 is live in the Store, and it looks to be a rapid-release fix for an issue affecting some users.
No other details were posted on the Xbox User Voice page, so it is safe to surmise no new features were added. In addition, seeing as this is a minor number change from 2.6.656.0 just a few days ago, this looks to be more about addressing a very specific bug for some people.
"A new release is coming for the Music app on Windows Phone 8.1 which includes a fix for an issue impacting some users launching the app. Version 2.6.657.0 will be available in the store later today.
"Many thanks to those who reported the error and to those who helped us verify the fix by sharing log files from their phones."
"Please continue making your feedback known on User Voice, in store reviews, and elsewhere on the Internet. We're listening!"
Regardless, hit the Store now and grab the update. The new version of Xbox Music is still very fresh, so give it some time if you do not see the update right away.
Source: Microsoft; Thanks, Dick S., for the tip!