If the speakers are connected to the computer soundcard you wont get sound out of them unless you switch the ASIO driver in the "VST Devices" window. Try listening through the headphone jack of the mixer. When you are using a device such as a mixer that has ASIO drivers it replaces all function of your computer sound card.
Also, what Tobe said. Check the output bus in the "VST Connections" window and make sure you've turned monitoring off on each track (the little orange speaker button).