Something about the sound of the Siren itself bugs me. Word of God is that, in the original game, it's the scream of Datatsushi. However, in the bonus cutscene, you can quite clearly hear Datatsushi's scream and the Siren song at the same time, and they're very obviously different sounds. So... how does that work?
The one where they're noshing on him? In that particular case, the Siren may have been for dramatic effect, although since D's an extradimensional alien, it's possible that his voice has some unusual properties.
So why did Tamon go to Hanuda? The guide says it's his hometown, but then why'd he bring Yoriko, who he doesn't seem fond of anyway? And who the hell is Akira? What's he doing there? Is he a native, or just some elderly hunter in the wrong place at the wrong time?
Hanuda is where Tamon was born and grew up until he lost his parents in an earthquake/landslide in 1976, when a failed attempt to summon Datatsushi caused Hanuda to be drawn briefly into the Netherworld. When he grew up, he became a folklorist, who returned to the area to do research on the local religion. Yoriko seems to have invited herself along against Tamon's will. Akira is a native, or at the very least someone who moved into Hanuda prior to 1976 (when he lost his wife in the same incident that killed Tamon's parents), and doesn't appear to have subscribed to the local religion.
If Hisako wanted to summon Datatsushi, why has she been carrying out the rituals for all these years? I thought the ritual was used to placate it and prevent it dragging the village to another dimension. Did she only decide to summon him after everything went to hell (literaly)? Am I missing something here?
The ritual is to revive Datatsushi by appeasing it. Hisako "ate the fruit", so she has to "give it back" in the form of her descendants every once in a while. Miyako was going to be the last one needed.
What was the thing in the morgue of the Miyata Clinic? I've heard it's a relative of Akira but having more details on it would be great. Also, how is it that no one has translated the Siren Maniacs book yet?
Well, according to a forum involving translations of the Siren Maniacs book, it's Akira's son, who interrupted the ritual in Hanuda in 1976, managing to save that generation's Miyako. Unfortunately, it turns out he was becoming infected by the red water, so he took a very Crazy-Prepared precaution; he had 1976 Miyako cover him up and tie chains around him before putting him in the morgue and driving the stake into his body. This way, he was able to save Miyako from the possibility of a shibito-him killing her.