Start by checking your Sound settings:
Settings > Sounds
If you don’t see an sound that matches the assigned sound, you’ll have to manually search through the Notification settings for each individual App - and identify those that have the ability to choose the individual sound.
Perhaps begin with Mail - as each mail account can individually configured in addition to setting a default sound:
Notifications > Mail > Customise Notifications >
- [Accounts] > Sounds - check for each configured Mail account
- VIP
- Remind Me