The hidden voices bug is fixed for next version.
But the investigation, each one gives you a lead that you can use against others if what someone said contradicts what other said.
I don't know if you still want to try to figure it out, if so, stop reading now.
Erica said she was reading under the tree, which couldn't happen because Emmet sprayed it with poison. Use Emmet's evidence on Erica.
Erica will then say she heard a man talking to Ami, but Ard claim it was a woman.
Use Erica's new evidence on Ard.
Hana said she didn't hear anyone talking to Ami, but now Ard's updated lead says both a man and a woman talked to her near the kitchen.
I think that's it.