Best answer by Airgetlam
So, since the app isn’t ‘playing’ the music, it shouldn’t be trying to access the speaker on your phone. In fact, to my knowledge, the app doesn’t even know that your phone has a speaker at all.
So, I’ve got a couple of generic suggestions. First, delete the app from your phone. Don’t worry, you won’t lose any settings, they are stored on the speakers, not the app. Second, power cycle your phone. Then, once it comes back up, reinstall the app, and test to see if it still has this behavior.
If it does, I’d call in and talk to a Sonos rep for further troubleshooting.