Cubase is not a video editing program. It is an audio editor that lets you play a video while you edit an accompanying sound track. Basically you edit your video in an external program, import it into Cubase so that you can watch it, and there either add completely different audio or edit the audio captured on the video. When you export from Cubase, you can only export the audio file, and then with external software you can join the two elements together. Google Audio Spot and check out their ER Media ToolKit program. It’s really good software and dependent on donations for payment.