Funny you should post this, I had an issue with our DVD player audio (see below) so I have some familiarity with this. I'm assuming you have a Jensen RV entertainment system. The unit is only held in with 4 screws and is easy to slide out. The wiring color codes are on a sticker on the unit. There should be a combo cable plugged in to it with yellow video, and left and right audio RCA connectors. The audio cables are actually audio-in, not audio-out. The video is an output. See if you can follow the video/sound cable, that might be easier said than done. Its also possible that someone at the plant forgot to run the cable. Oddly enough, every X18D photo I can find with the TV shelf does not show the A/V cable coming out. Here's a picture of what our TV shelf looks like with the A/V cable coming out of the wall on the right:
Our issue was different in that we want the audio to go to the TV so we can take advantage of the headphone jack. There is no headphone jack on the Jensen stereo/DVD player we have. Even with our old HTT, when it rains you can't hear the TV from the bed without really cranking up the volume. Our solution is a set of powered speakers mounted to a bar that snaps onto the shepherd's pole, so the sound is right above us. It works great. Too we don't disturb the neighbors with late night movies! I used a speaker to line level convertor to convert the unused "B" channel speakers to line level, then plugged the audio cables into that for line out.
2013 Jayco Jayfeather X20 E (sold)
2016 Chevy Silverado LTZ 2 Z71 Crew Cab (sold, and dearly missed)