Actually, they did add a new BGM in KMS, but it happened the patch afterward:
Personally I disagree about Kerning Tower, and here's why I think the storyline was complete garbage (and worse than FTW): It lacked any sense of tension, and plot points get resolved way too fast (for example, looking for Dia Mandy is built up to be this big conflict, only to get resolved in the same quest). And on the topic of that quest, Dia Mandy is just let back into the group instantly without any hesitation, despite it being hinted that her breakthrough performance was a disaster. And the appearance of the Spirit of Rock felt completely out of nowhere, without any sense of buildup.
Omega Sector was fine though.
Fantasy Theme World, while uninteresting, at least had a sense of conflict. Granted, I don't think anyone in KMS was really demanding that FTW have a new storyline, so it came out of nowhere for me too.