A number of items can only be used when their respective bar is full: lunch boxes, charging shields and jars come to mind. Those all produce a di-ding when they're ready to be used again.
Another such item is the dead ringer. I've been killed more than once by thinking the DR was full because all the pixels in the 'cloak' bar were white. How foolish!
Is there a reason why the DR doesn't make this sound? Is it an oversight from the developers, or has it been explicitly removed/not added for balance reasons?
Can I "hack" this signal in somehow, perhaps?