How to fade out/in in iTunes?
If you are using macOS, you can create a new Service with Automator, 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
                    repeat with i from current_volume to 0 by -1
                        set the sound volume to i
                        delay 0.01
                    end repeat
                    pause
                    set the sound volume to current_volume
                    exit repeat
                end repeat
            else
                set the sound volume to 0
                play
                repeat with j from 0 to current_volume by 1
                    set the sound volume to j
                end repeat
            end if
        end tell
        tell application current_application
            activate
        end tell
    end if
end tell


You can assign a keyboard shortcut to it in System Preferences > Keyboard > Keyboard Shortcuts > Services.