I don't think he needed to silence Hannah, sadly, if he did drug her. I think she'd have still been out of it by the time he got to that location, and he'd just have to pull her out of the seat, and take her where ever however he wanted to do so, drag carry, try to goose step her along. I think he just killed by strangulation or snapping her neck when he was done. I think this was an easy do for him.
MH, a whole other story. My guess would be she fell asleep in the cap or had her mind on other things as she was going "home", she told her friends, which is long way, and didn't pay much mind as to where the taxi was going. She was not fully capabel at the time anyways. My guess is that JM stopped, got out, and somehow got her out of his cab, whether with a well placed blow in the face that was unexpected. He's a big guy, and MH would have no chance against him. Once he punched her and dragged her out of the cab, he could just beat her into submission and drag her anywhere. When he killed her and how, we don't know but the autopsy report apparently has a COD according to Gil Harrington.
For Hannah, however, I don't think there was much screamining or running.