Does anyone know where I could find the OST of this horror game?

yeah, not sure the forum would like me mentioning it, but emule has what you're looking for. At some point, I may be able to submit the zipped album if you don't want to resort to emule. It's quite elusive though, you'd have to search global servers with the keywords 'siren ost' nothing else seems to work, and be prepared to wait some time. :) Hope that helps!