Does the hum happen all the time? Can you hear it at all when the jukebox is playing? If not, it may be possible that the mute is not activating when the jukebox is idle. check the orange with black stripe wire that is going to the CD mechanism and be sure it is making a good connection.
AIf the mute is working, try unplugging the RCA connectors going from the cd mechanism to the amp.Does the issue go away? If so, it may be a grounding problem. May be reaching for straws here, but you can always try a groung loop isolator.