Neither the Turks nor the Saudi's have (yet) requested any US help in investigating the Khashoggi investigation.
@Render already figure Turkey thinks it has substantial proof work that audio. Why would they?
Likewise with KSA working on a statement that absolves the Prince, I imagine they have accepted blame as much as they are going to/as much as anyone is.
@L_D_G Assuming that the Turks actually have audio of a murder inside the Saudi consulate they would be admitting that they bugged the Saudi's or have to stand by their claim that the victims phone and watch were doing something that those equipment models weren't capable of doing.