I'm running OSX 10.11.1 (15B42) on a 2015 rMBP 13". I have a Dell P2715Q monitor - I'm connecting this via a DisplayPort to Mini-DisplayPort cable to one of the Thunderbolt ports on the Macbook. My sound works fine normally. However, as soon as I plug in the monitor, OSX seems to disable the sound. If I try to adjust the volume, this is what I see: And of course, sound doesn't work in any apps (Chrome, Skype etc.). Any thoughts on what's going on, or how to fix it? Or how I can gather more diagnostics on what's happening? Could this be an OSX bug?