It's always been hard for me to add a music folder with subfolders to Windows Media Player's playlist. I double click a file in the folder (or click on WMP shortcut), WMP opens, and I drag the other files or folders manually to the playlist.Below is my answer:
Isn't there an option to add a right-click context menu item that can automatically add all audio contents in a folder (with subfolders) to WMP playlist?
In Windows 7 by default this option is available in the right click menu only for optimized folder for music:
You have to change the Folder Option - set it to Music (right click on the folder and select Properties and then Customize tab)
Now, when you right click on that folder you will see the option to add it to Windows Media Player.