OnePlus
Change Alarm Sound auf OnePlus
On OnePlus phones running Android with OxygenOS, the alarm sound is not only set when you create a new alarm entry. A separate ringtone field inside the Clock app settings can override whatever tone you picked earlier, which is why an alarm sometimes keeps playing the old default sound after you changed it.
This happens because the app stores a general default ringtone alongside the per-alarm choice. If the general setting was never touched, new alarms may fall back to it instead of a custom file you already imported.
To fix this, open Clock app ➔ Settings ➔ Alarm Ringtone ➔ Choose a ringtone. From there you can pick one of the pre-installed tones or select a custom ringtone or mp3 file stored on the phone.
If you only want a different sound for a single alarm rather than the default for all new ones, edit that alarm directly and change its ringtone field there instead of the general settings.
Side effect: changing the setting in Clock app settings updates the default used for alarms you create afterward, but it does not automatically change alarms that already exist with their own ringtone assigned.