Last week, Apple launched the second public beta build of OS X 10.10 Yosemite, which, among other improvements, includes a healthy dose of bug fixes. But, for some reason, the company has not yet addressed a glaring sound problem, where the audio stops working until a restart is performed. It is likely this occurs in the developer-only builds too.
I am not alone in experiencing this problem, as I have seen other OS X 10.10 Yosemite testers reporting the same issue with the built-in audio. In my case, after some testing, it appears that this issue occurs after my 2013 MacBook Air wakes up the built-in and external displays from sleep.
After the displays are on again, the audio control in the menu bar is greyed out. That also implies that the sound level cannot be adjusted, so there is no apparent easy way to recover from this bug. As I mentioned previously, a restart gets the job done, but this is a process I - and, I presume, everyone else who shares my grief - would rather avoid doing a dozen times or so per day.
After looking into the issue, I found a number of terminal commands which were said to bring back the disabled audio without having to restart the Mac. Of those, only one worked, namely "sudo killall coreaudiod" (of course, without the quotes).
You can type this is in a Terminal window (Terminal is found in the "Other" folder in Launchpad). The command is validated with your user password, which you will have to enter after triggering the command. Afterwards, the audio will work again.
This is not a permanent fix, as you will have to use the aforementioned command every time after the audio stops working. Still, it is a much, much more convenient solution than frequently restarting your Mac.