We’re sorry for the trouble and our teams are hard at work to resolve this. We'll post updates here as soon they become available.
In the meantime, if you have an alarm set for tomorrow that is going off and can't be stopped, you'll want to go into your Sonos controller and select Alarms. Then, disable any alarms scheduled daily or including Saturday. If disabling the alarms doesn't do it, deleting the alarm will.
Once that's done, you can stop your music as normal.
Best answer by John M
Earlier today we discovered a bug in how some Sonos players interpret leap year dates, resulting in unexpected behavior for alarms set through your Sonos speakers. We're sorry for any inconvenience this has caused.
Until the end of the day on January 1st, 2017, alarms set on Sonos may trigger on the wrong days or times, not trigger at the time you set, or repeatedly loop / not turn off.
For the rest of this weekend, we recommend you disable any existing alarms. If your alarm is going off and can’t be silenced, disable the alarm and then stop the music. You’ll find instructions for alarms here.
After the 1st of January, we expect all alarms will resume their normal behavior. You have our commitment that we will fix this bug in a future update.