OnePlus
Change Ringtone for Calendar Events and Reminders auf OnePlus
On OnePlus phones running OxygenOS, calendar reminders use their own sound setting separate from the general notification tone. If every alert on your phone sounds identical, it usually means the Calendar app is still using its default sound instead of a custom one you have chosen.
To change it, open Open Calendar ➔ Open the menu ➔ Open the Settings ➔ Tap on General ➔ Tap on Calendar notifications ➔ Tap on Advanced ➔ Tap on Sound ➔ Choose a ringtone.
From that sound picker, you can select any of the ringtones already installed on the system, or pick a custom audio file such as an mp3 stored on your device. This setting only affects Calendar app alerts. It has no effect on reminders created through other apps, such as a separate to-do or task manager, since those manage their own notification channels independently.
If you do not hear the new tone after saving, check that Calendar notifications are still allowed under the system notification settings, since a disabled notification channel silences the sound regardless of what you pick here.