In order to customize the sound notifications, please follow these steps:
Settings | Notifications | [select the account] | Incoming Sound. This opens Android's own per-app notification settings, where you can pick the sound, along with vibration and other channel-level options.
Note: Check you are not in silent mode, when checking sound notifications. Also, Android is able to play only one sound at a time, so if you receive notifications on your emails from more than one apps, only one sound will be played.
