Think a bit more info about what you've tried already would be useful. It could be any number of things... the monitor system depends on what you've selected with the Monitor Select button. If it's set to anything other than Stereo, you may not hear anything. Does any sound come out of the stereo outputs, is it only for songs playing back from the card, does it play the demo song, have you tried a signal direct from the inputs, have you tried a factory reset, etc.