According to
foo_thbgm, a Touhou decoder for Foobar2000, Tasofro has been using esoteric XOR-Mersenne-Twister-based encryption for their data files so far, and it has been said that the TFPK format used in HM is not much different from the previous formats, so it's not surprising that Audacity can't decode it. Unfortunately, the aforementioned GitHub project has the TFPK unpacking algorithm "censored" out (by having the relevant inline-included file excluded from the code repository), and the person maintaining the project has not responded to my E-mail asking about the missing file. The resources themselves are in the OGG/Vorbis, Bitmap and DirectDraw-surface formats, according to some old extraction of the first HM trial I can't seem to be able to find right now.
My theory about the two different .pak files is simply that the "b" file includes overrides for Koishi's previously crashing moves (among other things that were fixed), to avoid the need of either replacing the th135.pak file itself, resulting in a massive patch size, or having to byte-patch the th135.pak file, resulting in worse version skip compatiblity for subsequent patches.
If anyone knows more about the algorithm (particularly those people with links to the Chinese community that figured out the formats), I would
really appreciate some information on how the TFPK format works. As I said elsewhere on this forum, I'd love to be able to listen to HM's music outside of the game or its musicroom.