Quoted from PinMonk:
Check the cable inside the cabinet at the front, and if that's connected, open the box in the head with the computer motherboard and make sure the audio connection is plugged in there and didn't come loose.
Yes it is going to be either one of these two places to check. If you need to get in the back here are some pics and tips The monitor is in front of some of the computer goodies you need access to. There are some wooden lock in supports you need to loosen in order to swing it out. I hope this helps.