- Online and Offline mode
Folders may have several options configured individually per-folder.
To open folder preferences, click on arrow on right side of folder item, and choose Preferences from context menu.
You can configure folder specialization, number of messages to download in the folder, notification sounds.
The options are well described in the preferences dialog.
Folder's sound for new mail notification may override default sound in upper level.
See how Notification sound may be configured here.