So the days and recoded quotes are easy, they're stored in what's called the sdat files in the snd directory in the ds disc. You can get an sdat decompressor, and this spits them out as wavs (i think the extractor has an autoconverter) - as a note you can get all the music for this game in a format analogous to midi (no instrument info), and then you can also get bank information which contains the instrument sounds - this means that for example destati (which uses a real choir,) the wav sample will contain the choir recording, and the notes used in the midi analogue, won't correspond to the notes the choir are singing, it will just refer to a note e.g. F, and that note will be map to the choir singing 'forza tendi la mano'. That is only true for pre-recorded samples including the more complex percussion, lyrical choir, and the saxes in traverse town and lazy afternoons
The exception to the midi analogue is the streamed tracks (DB, Passion/Sanctuary/S&C, etc) and music that's used in the pre-rendered cutscenes .
The BBS voices were a lot harder. What I did was unpack the game; when I was in another forum, nobody could find the voice files. I noticed there was a file called SSCF (which I thought sounded like it could be a voice container), and ran it through a program that could find wav headers, and it turned out that each sscf was like a container for character voices, sfx, and battle dialog. However they were all sped up, so had to use audacity to try and get them to sound normal.
I think I asked Corey Burton once, how dual voicing was done, and he said that typically it would be the lead of who recorded the dialog first, the second person would then listen and have to match them with help from the direction. (I think that of the Ventus-vanitas merged voices, ven is very hard to hear.)
