I've put together a package to automate the decoding of the MI2 music.
Instructions are in the readme but the process is basically this:
Download it here.
Drag and drop an .xwb file onto the file named DECODEAUDIO
It'll take a few minutes to complete, the new files will be in the "Decoded" folder.
I've only tested it with MusicNew.xwb as my brain is fried, but I assume it'll work for some of the others.
See the readme for more information.
[Edit] Updated the download with a new version that is a lot quicker (thanks to Meelkee)
Use the latest version of Monkey Island Explorer