the Nvidia is your video card, so it will send sound through the HDMI cable. Try disabling that option in the Sound menu. Set you default audio to one of the other ones on the list and test. Check your Device Manager and make sure you have the sound card driver installed. Verify you have the green cable connected to the right port. Try connecting the green sound cable into your motherboard and see if you can get sound from there, instead of using the sound card. Make sure your speakers are turned on and sound volume is up.