Recently, I came across an issue that I ignored for as long as possible, but not being able to adjust volume level can be a bit frustrating to say the least.
The issue that I was seeing is captured in the screenshot above. The volume level appears to be all the way up, but is actually disabled.
The fix that I’ve found, which is by no means a permanent solution, is to plugin a set of headphones and then remove them again. For whatever reason OS X seems to re-enable the audio driver and sound is restored!