If you want to refer to files/folders in the music package, you refer to them with nothing but their file/folder name, just like you'd refer to files in the data package. Like this:
if(!Game.IsMusicChannelPlaying(0))
{
Game.PlayMusic("dest2.ogg",true);
}
So IF you had a folder called music in EITHER data OR music package, only then you'd have to use the folder name. As far as I know, there's no way of referring to packages directly.
And if you have dest2.ogg in both music and data package, the file in the package with the highest priority setting is used.
Also, if there was a different way of referring to different packages, it would sort of defeat the purpose of having packages. For example, making bugfixes with packages would be impossible, because there would be no way of replacing files.