My alarm clock does not work

Try this:

If you have an alarm that doesn’t sound or is too quiet, or if your iPhone only vibrates, check the following:

• Set the volume on your iPhone. If your alarm volume is too low or too loud, press the volume button up or down to adjust it. You can also go to Settings > Sounds & Haptics and drag the slider under Ringers And Alerts.
• If your alarm only vibrates, make sure that your alarm sound isn't set to None. Open the Clock app, tap the Alarm tab, then tap Edit. Tap the alarm, then tap Sound and choose a Sound.
• If you use iOS 11 or later and you connect headphones or speakers to your iPhone, the alarm plays at a set volume through the built-in speakers on your iPhone as well as wired headphones and speakers. It doesn't play through wireless headphones and speakers.

Read more here:

https://support.apple.com/en-ke/HT207512