To set an account specific sound (and other notification options), please:
- Long press on the account on the app's main screen
- Choose Special settings
- Tap into "Special message notification settings for this account", enable, set the details
What folder? The app doesn't care, it uses the standard Android mechanism(s) for picking the sound.
I think some (not all) file manager apps can "inject themselves" there so you can pick a notification sound from anywhere on the file system.