No sound with yosemite

Question

have no sound for my downloaded iTunes music after installing yosemite on my macbook air

Answer 1

Back up all data before proceeding.
Triple-click anywhere in the line below on this page to select it:
~/Library/Preferences/com.apple.iTunes.plist
Right-click or control-click the line and select
          Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder should open with an item selected. Quit the application if it's running. Move the selected item to the Desktop, leaving the window open. Relaunch the application and test. If there's no change, quit again and put the item you moved back where it was, overwriting the one that may have been created in its place. Otherwise, delete the item you moved. If possible, restore it from a backup that predates the problem.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination  command-C. In the Finder, select
          Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.