Hello, my name is*****'m a sound engineer, I work with inputs and outputs on computers, all the way up to large concert speaker systems. There are 2 places where the audio output can change in your computer when Cubase is open.
1. Go into your input/output (I/O) settings in Cubase. Choose your output device, and select the outptut you want to use. These are selectable, so if you change something you can always change it back.
2. Your Windows also has input/output settings. Right click your speaker icon near your clock, and select PLAYBACK DEVICES. Choose the same one you chose in Cubase, and click SET AS DEFAULT. This will output your sound to the device you choose.
Now would be a great time to get used to routing your inputs and outputs. When you get an interface, you'll need to set the inputs, and if you plug your speakers into the interface (recommended for audio quality), you'll want to set each of those settings to your new interface.
Audio can be routed, you just have to tell it where to go. The I/O settings in Cubase and right clicking the speaker icon near your Windows clock are the two places where you route input and output devices. Let me know if you have any trouble.