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.
For what BlueMail supports in the first place, see Does BlueMail Support Notification Sounds?.
