I thought all the music is available through bonanza now? (Well, except the seasonal ones.)
They are somewhat not well known ones because the reasons I mentioned and also because they don't show up as blanks. From what I can gather (info is a tad sparse) these tracks come from the having certain furnishings out. Spinet which came with buying old soundtrack or rainbow pell which involved buying other random irl stuff. Nanaa Mihgo Statue which is from another CD or rainbow and Nanaa Mihgo S. II which again rainbow and apparently going to a concert.
Like a lot of the ig items attached to irl these are generally only available thru jp sites some of which wont even let you try with a non jp IP and/or address. And some of the ones that do wont mail you the code even if you can order it and even if you try to go thru 3rd you still often don't get code and several only offered the code for limited time anyways or the item sold out fast. THere's actually quite a few items like this that most non jp players don't know about. Like pretty much everything on the rainbow pell and maybe even some other pells we don't see often