Okay, first I would make sure that the MIDI data is getting back into the Triton. Then I'd make sure that the audio is actually leaving the Triton and going into the audio interface. I'd make sure the audio interface can accept incoming signals. I'm make sure the audio interface is set as the default thingy in Windows Control Panel. I'd make sure the audio interface's inputs are set as the primary inputs in Cubase.
Basically, you may want to check out this article:
Soundcard Troubleshooting 101
Brandon