In our PC we already have an inbuilt sound Card, for audio output.
1- Why do we need a separate audio output (SPDIF) in graphics cards
2- How much is the voltage in SPDIF cable?
3- Can I run the graphics card, without connecting the SPDIF cable, taking the video output from graphics card and audio output from the inbuilt sound card?
SPDIF is for sending audio to a tv using a HDMI cable, if you have normal speakers you can use your sound card, if you only have a tv with inbuilt speakers you might want to send the audio over a HDMI cable and for that you need to use the graphics card to do so.