That's what you get for going through a mountain of old cases while doing a few things on the current one at the same I guess. If the board lacks a beeper it may be a model that displays information on the first post screen in the event of any hardware failure like a bad cpu or memory problem.
For adding a small speaker you buy separately however you still have to look over the diagram seen in the manual or open the downloadable version from the support site. Depending on board sometimes there are no color coded areas for the different pairs of wires for power ,reset, leds, etc. where the diagram will point them out. Verifing which sees what wire can be fun at times since you are working in a tight space.