No sound playback thru PC's ext speakers, Cubase LE 4

Hi, I hooked up a Lexicon Omega interface to my PC via USB and I am using the Cubase LE 4 software that came with it. My PC uses Microsoft Vista OS. I have followed the instructions to select Omega ASIO under VST Audio System and Devices in Cubase LE 4. Under my PC’s “Manage Audio Devices” in the Control Panel, I have selected Speakers, Realtek High Definition Audio under the “Playback” tab and “Stereo Mix, Realtek High Definition Audio” under the “Recording” tab. I have followed the settings as given by the Omega Manual for voice recording via a dynamic Mic plugged into the Mic 1 XLR jack on the back of the Omega, and assigned USB 1-2 for Mic 1, and USB 1-2 for the Monitor Mix on my Omega, and I can get a playback through the headphones jacked into the Omega itself. I also followed the instructions as per the Omega Manual to begin a New Project to record a track through Cubase LE 4, but my problem is that I cannot get any sound to playback via the external speakers connected to my PC’s Realtek soundcard. Also, when I try to record a “live” vocal track with Cubase, I notice that the graphical Audio Monitor in Cubase does not display any recording signal, i.e. it appears that no recording signal appears to be received by my PC. I have also imported a pre-recorded audio file or mp3 into this New Project and when I play it back, I cannot hear any sound from my external speakers. I have also tested my speakers with Windows Media Player and the speakers are working fine. So, what am I doing wrong? What am I missing out? Please help!

1.the windows audio settings are irrelevant for Cubase. Cubase uses ASIO and has its own playback and recording settings.
2.cubase will playback via the device, which device is set as driver in Cubase.
3.All that is very well explained in the Cubase manual and more than once on the forum here. Just like how to set up busses to receive an input signal in the VST connections page.