[strike]That should theoretically be possible, since the monitor seems to have a USB hub built in (you'll probably need to run a usb cable from your PC to the monitor as well). I don't see what advantage it has over connecting speakers directly to your PC motherboard, though.[/strike]
Edit: I misunderstood what you were asking. The USB ports on the monitor only act as a hub, they don't provide expansion capability for things like usb sound cards when hooking up a Roku. The Roku 1 has RCA audio outputs, though, so you can get an adapter for those to 3.5mm TRS for PC speakers.