That depends on the format of the file(s) you play, and on what you mean with 'join' and 'separate'.
As for you last question: video isn't amplified and audio isn't displayed on the monitor. So that question is confusing or wrong. Moreover, what do you mean with 'amplifiying' a digital signal?
Hi Cnet forum. Is the video and audio of a file going to an SD card joined together? When being read from the SD card, the video and audio is still joined together until separated in the computer? Is the video and audio once separated amplified before being displayed on the monitor?