If your soundcard has ASIO drivers then you can run a low-latency monitor mix out through Cubase including the input signal(s). If not you'll need an outboard mixer of some sort - run the output from Cubase into one channel, your input(s) into the other(s) and create your headphone mix there.
But TBH it would be just as cheap and probably less faff to buy a decent audio card and stop using your soundblaster. In fact, is the Yamaha 'thingy' an audio interface? Is that where you're getting your headphone signal from? If so you should be able to disable input monitoring in Cubase and set up the mix right there on your audio interface.