Hmmm. That's strange.
I corrected the problem by reinstalling the mod and latest patch, but there is certainly a folder "XML" within the music pack, unless I did something terribly wrong. It contains CivLeaderHeadsInfos.XML and CivEraInfos.XML. Reinstalling the music pack again on the latest version again gave me the errors. I remedied it (I think, and I'm no expert) by again reinstalling the mod and the patch and manually dropping the music files into the correct directories (and not doing anything with the XML files).