Yesterday I ran into a new Yosemite feature that annoyed me a bit. After changing the input on my Dell 27" display from DisplayPort to HDMI, the screen turned black on my 27" iMac, and audio stopped. Forcing a reboot (holding the power button for >4 seconds) was needed to get the iMac's display back.
But from that point on, the audio was greyed-out in the menu bar. Changing the volume on the (Apple) keyboard gave a disabled icon on screen. Also, no audio was playing over my external speakers.
My first thoughts were that the earlier crash had corrupted something on my system, so I did an additional reboot. Nothing. After that a PRAM reset (power off, power-on and hold command-option-P-R until you have heard two start-up 'boings'). The start-up sounds were there, so the actual audio hardware was just fine.
When the desktop loaded still no audio control, until I unplugged my DisplayPort connector on the Dell 27" monitor. Audio (controls) came back instantaneous.
So, with DisplayPort connected to the external monitor: no audio (controls), and without the DisplayPort: audio (controls).
Turns out that with Yosemite, the audio is channelled BY DEFAULT over a DisplayPort connection (to a external monitor). In my case, the Dell also has an audio out connector, and I guess that is 'advertised' over the DisplayPort.
Changing the default behaviour is done in the System Preferences -> Sound
The first image shows the default (at least in my case). Changing the settings to the second image gave me back the audio and volume control.
I have no idea if this was also possible with Mavericks (or even earlier versions of OSX), but it's definitely a (default) feature that annoyes the hell out of me.
Even though I tackeled the audio problem, the issue with loosing the display when I change the video input on the external monitor still remains. But only if the desktop is extended to the second screen. It doesn't occur when the screen is mirrored.
;)