iPhone alarm goes off but makes no sound

Edit the alarm and check what it says next to Sound. If it's set to None you only get vibration, and it's easy to pick that by accident. Alarm volume follows the Ringer and Alerts slider in Settings > Sounds & Haptics, not the volume you set for music or videos. If that slider is near the bottom, the alarm is near silent. The silent switch doesn't matter for alarms. Also worth testing with Bluetooth off for a night, in case it's going to earbuds or a speaker you left connected. And if it's the wake-up alarm from your Sleep schedule, that one has its own sound and volume settings in the Health app.