Philips 22RH457 speaker box, low sound level

I have two boxes Philips 22RH457. One gives normal sound level, the other has markedly lower sound level. For a quick check on anything different I measured impedance of both simply at the connection cable. Indeed, the normal one 6.5 Ohm the lesser one about 55 Ohm. Before starting to open the box, could anyone give an idea of the probable cause. The deterioration has been gradual because before both produced the same volume. Perhaps speaker failure, some condenser failure, etc. Please let me know. Many thanks in advance!