From what I've gathered through various fan communities and official channels, '瘴気の谷' hasn't released a standalone original soundtrack yet. The atmospheric tracks from the game often get discussed in composer forums, with many hoping for an eventual physical release.
Interestingly, some of the bgm tracks can be found scattered across different platforms - a few battle themes appeared in limited edition bonus discs, while environmental tracks sometimes loop during livestream events. The composer's Twitter occasionally teases unreleased pieces too, fueling speculation about potential future releases. Until then, fans keep compiling unofficial playlists from in-game recordings.