Why does Apple's mobile phone alarm clock only vibrate without sound?

Because there is no alarm clock selected, the steps to turn on the sound are as follows:

1, turn on the alarm clock.

2. Click "Edit".

3. Click the alarm clock you want to edit.

4. You can see that the ringtone is "None" at this time, and click "Ringtone".

5. Check the desired ringtone and click "Back".

6. At this point, you can see that the ringtone is a "constellation" and click "Save".