Here's what we know regarding who did what for the music in the initial release:
- Iwata composed the title theme, according to an interview on his old English website. He does not necessarily confirm or deny composing anything else, though.
- According to Iwata in the same interview, Sakimoto composed "3 tracks: the opening movie, field music, and another piece". There are multiple field themes so I'm not sure which one, while "another piece" is very ambiguous and could be anything".
- Namiki only composed the other building theme, which I verified with him. (he referred to it as "shop BGM", although a different track is used for that purpose as far as I know) He finds the Hollywood-style music that is common in JRPGs boring, which is why he went for a minimalist/chamber-style approach.
No info about what Koyano did, but judging by the info we have above, as well as the rompler-ish quality of much of the soundtrack, it appears he was the main composer. While he was never part of Basiscape, it seems that they worked with a few outside contractors (e.g Yasushi Asada) to help fill out soundtracks until around 2007, when Kudo and Chiba joined.