/usr/share/kde4/apps/mail_plasmoid/mail_plasmoid.notifyrc is in plasma-widget-mail 0.3.1-0ubuntu7.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 | [Global]
Comment=mail plasmoid
[Context/group]
Name=Group name
Comment=The name of the group for contacts
[Event/newMessage]
Name=New E-Mail
Comment=You've recieved a new email.
Sound=KDE-Im-New-Mail.ogg
Action=Sound|Popup
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