The notification sound is played when new mail arrives.
There are more levels where notification sound may be configured:

• Device’s default notification sound set in Android’s system settings is primary sound played when no other configuration is made
• This may be overridden in Configuration for entire ProfiMail
• Then, Account may specify sound played for mail retrieved by that account, this is configured in account preferences
• Finally, a Folder may specify sound to play when mail arrives at that folder, this is configured in Folder preferences

So it is possible to fine-tune notification sound on various levels. Leaving the notification as default will use sound notification from upper level, until some non-default sound is found.

Valid sound notification setting is also Silent, which means that notification sound will not be played.