This does seem odd. I’m assuming you’ve rebooted your router, followed by rebooting your speakers to pick up any potential DNS changes between your Sonos and the CBC server? I’d also check your Sonos system for any updates you may have missed, using your controller.

However, if none of that made a difference, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

The channels miraculously reappeared this morning. No issues (until the next time it happens). It might be as a result of updates.

Or, it could be an issue with the server owned by CBC that serves the Sonos stream, we don’t know. 

CBC Radio 1 Toronto is my most-listened-to station, several times a day. I haven't experienced any issues recently, including in the time frame discussed here. That's a welcome relief from the early days of the new app, when all CBC stations would regularly disappear or not play.