Once you've installed the latest drivers for your Line 6 USB device on your computer's operating system, connect audio monitors/speakers to the audio outputs of your Line 6 USB device or use the headphone output jack.
The built-in soundcard output of your computer will not output processed amp/effect tones from GearBox/POD Farm/DAW software. Also, connecting the audio output from your POD Studio/TonePort/POD interface into the audio input of your default soundcard will result in you hearing latency (short echo/delay). Please connect your computer speakers or headphonesdirectly to the output of the POD Studio/TonePort/POD interface device and not your original computer's soundcard output.