I have a Mac Mini (late 2012) running the currently latest osx, El Capitan, and as display I use a Dell U2713H connected via thunderbolt (to a DisplayPort on monitor). This configuration is needed to get the maximum output resolution of the monitor, but it seems like it disables the volume control somehow.
When accessing the Sound panel of the system settings, it has disabled the Sound off/mute button in the bottom, and on the Output tab it says (in Norwegian): The selected device has no output controls. And the volume control (on keyboard) doesn't do a thing, neither for the 'global' volume or when playing stuff in iTunes.
My actual audio output is connected through the monitor through to a set of external speakers. The reason for this connection is that I also use the monitor as TV display by utilising the HDMI port on the monitor.
Do you have any suggestion on how to re-enable the volume control with this configuration, if at all possible? Or any alternate solution to enable be listening to sound (not simultaneously) from both the Mac mini and the TV with only one set of speakers (no dual input available on the speakers).