Cubase 5 Output to WebCam Input

Hi there, this may be an unusual question.

Interface: M-Audio Firewire Solo.
OS: Windows 7.
Software: Cubase 5
Note: I run all computer audio through my interface - all other audio devices are disabled.

I downloaded some free webcam software (ManyCam 4.0). I’m able to select “System Sound” as the audio input into ManyCam, meaning I can record videos with the audio coming out of my interface. I am able to record any sounds coming out of my speakers (songs, videos, youtube, etc), except the output from Cubase 5 (even though it comes out of the same interface).

The reason I want to be able to record Cubase 5 output in my webcam videos, is because I run my guitar amp directly into my interface, then use amp cabinet software to simulate a cab. I then click “Monitor” so that I can hear what I’m playing. I want to capture this processed sound in my webcam videos live.

Since everything is coming out of the same output of my interface, why would the webcam software pick up everything except cubase 5?

Thanks for any assistance.