Hi Garden-Lady,
Welcome to Apple Support Communities! We understand that you're experiencing issues with the alarm on your iPhone 12 having no sound. We're happy to assist.
You mentioned checking the volume on your iPhone. We'd also recommend giving the steps under the "If the alarm volume is too low or quiet" section of How to set and manage alarms on your iPhone a try.
If you continue to experience issues with the alarm, let us know if the issue persists if you create a new alarm, or change the alarm sound. This information will help determine our next steps.
Cheers!