1) First of all open up terminal(use Ctrl+Alt+T shortcut) and execute next command:
sudo gedit /usr/share/glib-2.0/schemas/com.canonical.unity-greeter.gschema.xmlType in your account password and hit enter.
2) It will open up "gedit" text editor, scroll down and search for "play-ready-sound" line. Now under that line change the value from "true" to "false". Save the changes with "Save" button at the top of text editor.
3) And now execute the command given below in terminal:
sudo glib-compile-schemas /usr/share/glib-2.0/schemasNow you can restart your computer to see the results, you will no longer hear the drum roll sound when logging in to your user account.