You don't tell us what mobo you have, so I can't answer this question: Does you mobo have its own built-in graphics chip?
Reason for the question is this: IF you have graphics on your mobo the BIOS will be set by default to output graphics via its own chip. To get your add-on GTX graphics card to be the output device, you must go into BIOS Setup and change the setting. Until you do that, you will have to use the MOBO's graphics output connector (of whichever type) to connect to your TV so you can see the display.
A similar situation will arise for sound. Windows can only use ONE audio output device at a time, and I expect it (not your BIOS) will be set (once Windows is installed) to output audio through the default mobo audio chip and its connectors. BUT if you want audio to be output on the HDMI cable coming from your new graphics card to your TV, you will have to change Windows' setting. The graphics card cannot pick up an audio signal from the mobo audio chip. It has its own audio output device built into the graphics card itself. So you will have to tell Windows to change to using that audio output device (in the GTX card) as its sound outputter.