If you are missing the Alarms tab in the iPhone iOS app normally found in Settings, here is one possible reason and solution:
Location permissions is off therefore the Date & Time time zone is not set. This will result in the Alarms tab disappearing from your system settings.
You can either set location permissions to either “while using the app” or “always on” in your iPhone settings OR manually set your time zone, within the Sonos app, in the Date & Time tab found under Settings/General Settings/Date & Time.
It took me hours using Chat support and Phone technical support (including uninstalling and reinstalling the app, restarting my iPhone, trying to set up Alarms on my laptop Sonos app (which did not work), sending diagnostics, amongst the many other failed attempts to resolve this issue) before I discovered the solution myself.
Hopefully this will save someone else hours of grief.