How to measure a hex or jam nut?
I would first check that the missing hex nut is the same size as nut on one of the two output jacks. This can be done by removing one and seeing if it fits on the jack with the missing nut. This temporarily removed nut can be used as a sample for comparison when buying a new nut.

Nuts of this style should be readily available via online retailers such as Digikey or Mouser. If you go to a retail store it may be harder to find the nuts as an individual purchase item. You may find yourself purchasing a whole replacement jack and stealing the nut from it to replace the missing one on your speaker. Some of the really good (often older) hardware stores will have specialty drawers with various odd ball fasteners and nuts. You could get lucky and find your replacement nut there.

http://www.mouser.com/catalog/catalogusd/647/2952.pdf