I'm running a MBP in clamshell mode connected to a 24" ACD. However, no sound is being played through the speakers on the ACD. The speakers have always worked, but for some reason, I was just testing them right now and they no longer work. The sound is being played through my MBP's speakers and I can only control the volume on there and not the ACD. The brightness controls work fine. It can't be the cable then, can it? The volume popup shows, but it's controlling the volume on the MBP and not the ACD. Any ideas?