So did I after installing OSX Yosemite.
Turns out that OSX Yosemite changes the sound of feedback when changing volume levels on your Mac and defaults the change to not play the feedback.
Simple fix if you still want feedback when changing audio levels.
Open System Preferences (lots of ways to do this, including using Spotlight search).
Click on the sound Icon in the second row on the right, or use the search bar to find it:
Then, in the bottom third of the window, locate the check box for “Play feedback when volume is changed” and make your selection:
That’s all there is to it!