I have both 32-bit and 64-bit C5 installed, but I’ve only recently started to use 64-bit exclusively. I launched it tonight and can’t get any sound out of it (I see activity in the meters, however). Connections (under “devices”) seem fine
What am I forgetting
Turn on the monitors? 
Seriously though… it’s gotta be something simple. Like I’ve run into that problem before, because I didn’t have my headphones plugged in. (I unplug them before shutting down because there is a pop when the ASIO driver cuts off on the shutdown routine).
If not… what is your basic setup, from Cubase / computer on out?