Actually, you can continue to use your PC's soundcard and computer speakers for listening to youtube (if you want). You just need to select the PC soundcard as your audio device. If you leave the power off to your interface, then the computer should 'choose' the soundcard by default, then when you kick on your DAW, turn on the interface first, and if you have selected the interface as your audio device in the DAW, it will automatically choose it.