It is now fixed with the latest BIOS update...[URL] 
I have discovered an odd issue related to the Dell 5420's and am wondering if anyone else has come across this.  If I take a brand new laptop and plug an external monitor via HDMI or DP, then turn the laptop on and close the lid, the monitor is not detected as a sound device.  If I repeat the process, but leave the laptop open the audio comes through the monitor just fine. The issue is, I have found this to be true with a Dell docking station as well (Both WD15 and WD19).  If I dock a powered off 5420 with the lid closed, and let the dock turn it on, I am unable to get any audio out of the monitors.  If I open the lid right after docking, the audio works just fine. So far I have tested this on 5 laptops, some imaged with our company image, others straight out of the box.  I have updated all drivers and firmware, installed windows updates, installed monitor drivers attempted with HDMI and DisplayPort but get the same result each time.  If the lid is closed, when looking at the device manager or  the Playback sound settings the audio devices do not exist.  While so far this has only affected a few of our users that use monitor speakers, I think this will become a much larger issue as we deploy more laptops.  The current resolution for our users is to open up the laptop each time they reboot, then close it after they have reached the login screen.  Easy, but quite annoying.Any verification or help with this would be greatly appreciated.Alex
Dell 5420 No HDMI/DP Audio when laptop is booted while closed