Well actually there's only one way to get sounds playing from a pk3 file, which is by loading them with the ubersound file. This means you can't use the .mus file

.
Really annoying isn't it? I will keep looking for other solutions though.
To learn more about the ubersound file, read this:
http://www.planetmedalofhonor.com/map/t ... sound.html
It's really the developers fault that this doesn't work. All the ubersound crap should have directly been shipped into the trash bin.