I'm not familiar with Cubase, but some on here are. You might have more luck if you could describe your problem in greater detail.
What I think you are saying is this: When you try to record a punch-in to an existing take, you can either monitor the input from your microphone, or hear the previously recorded take, but not both.
If this were Reaper, I would suggest looking more deeply into you track sub-menus. You might be looking for a function such as "overdub" which would allow you hear both the recorded take and the input. I'm sure it's just a matter of finding the right toggle to click.