Attention: When the Casio CZ-230S is disconnected from power for some days with no batteries inserted, it looses its RAM contents and by this often crashes so deeply that all preset sounds or at least the sounds 96 to 99 stay mute. Sometimes it even turns itself off when starting a rhythm. To fix this, simply reset the instrument (erases memory contents) by holding the red "C" button during power-on.
This was suggested but seems the C button when hold does not reset.