How to restore the Mavericks version of the sound effect for the volume button for Yosemite?
It can be done, although you need to find the Mavericks volume feedback sound file. The filename is "volume.aiff" and (for both Mavericks and Yosemite) it is located in:
/System/Library/LoginPlugins/BezelServices.loginPlugin/Contents/Resources

Find the Mavericks volume feedback sound file and put it on your Desktop for easy access.

Open the Terminal and navigate to the folder containing the sound file:

cd /System/Library/LoginPlugins/BezelServices.loginPlugin/Contents/Resources


Create a backup of the original file in case you want to reverse the changes:

sudo cp volume.aiff volume.aiff.bak


Provide the password when prompted.

Now copy in the Mavericks sound file placed on your Desktop:

sudo cp ~/Desktop/volume.aiff ./


Restart