Today I changed the setting in Sound in Preferences to the external monitor and video and audio worked perfectly. So that mystery has been solved. However, when I close the lid --clamshell mode -- video and audio disappear and monitor says no hdmi connection. Just for reference, I'm running this test with no external keyboard or mouse etc connected, and no power cord into rmb. Apple says that is mandatory in order to be in clamshell. But shouldn't I be able to be in clamshell without kb-mouse/trackpad and power connected? It would seem so, but it does't seem to work just standing alone. Any thoughts? TIA