After too much time spent on Google, forums, moving the SmartThings hub to different ethernet switches and trying all kinds of router/network configuration options to no avail, the SmartThings iOS app finally detected my Sonos speakers with this counterintuitive process:

1. In the SmartThings app, click the + on the main screen.
2. From the “What do you want to add to this location” list, select Device.
3. On the “By device type” tab, select Speaker. All obvious so far.
4. The only option available on the Speaker screen is Bose. You do not have a Bose speaker. Click Bose anyway.

My non-Bose Sonos Arc and other devices were magically detected by the Bose button. Give it a try.