The resource bar was replaced several builds ago. Now there is this:
You can add sounds here. The Sounds and Music folders aren't finished yet, so if you want to add sounds to the .exe and play them with XA2 as a resource, you need to put them in the Files folder. Right-click and select "Add file(s)"
As for playing sounds from a file using AppPath, you need to use a backslash, not a forward slash.