iPhone alarms unexpectedly play loudly on Bluetooth speakers

This can happen if the Bluetooth speaker is still the active audio output when the alarm fires. In that case the alarm follows the current audio route and plays through the speaker, often at full alert volume. Most speakers will turn off automatically after a certain time, assuming you have a timer on the music or similar, then the phone would fall back to the built-in speaker before the alarm triggers.

An alternative is setting an alarm that gradually increases in volume. You can create your own via Garage Band or with some third-party apps generate alarms/ringtones with gradual fade-in (for example Rising Tunes app).

• If you connect headphones to your iPhone, the alarm will play at a set volume through the built-in speakers on your iPhone as well as wired and wireless headphones.
• When you use StandBy, haptics for your alarms are disabled. To make sure that your alarm sound isn't set to None: open the Clock app, tap the Alarms tab, then tap Edit. Tap the alarm, then tap Sound and choose a Sound.

Manually adjusting the time on your iPhone in Settings to change app behaviour, like game timers, isn't recommended and can affect alarms.