In Automator, create a new Service, add a Run AppleScript action to it, and enter the following script:
set current_application to (path to frontmost application as Unicode text)
tell application "System Events"
if process "iTunes" exists then
tell application "iTunes"
set current_volume to the sound volume
if (player state is playing) then
repeat with i from current_volume to 0 by -1
set the sound volume to i
set the sound volume to current_volume
set the sound volume to 0
repeat with j from 0 to current_volume by 1
set the sound volume to j
tell application current_application
You can assign a keyboard shortcut to it in System Preferences > Keyboard > Keyboard Shortcuts > Services.