I turned it off because it was "spamming" me with information about plasma sound issues, but it appears that without it system also doesn't mount usb storage devices automatically. How to bring it back?
If I understand your problem correctly, you might have remove the Device Notifier widget.
To add a new Device Notifier widget to your panel you need to right click on a the far right element of the pannel and select Add Widgets ... from the presented menu.
Then select Device Notifier in the widget list and click on the button labeled Add Widget.