When I play these videos on my computer, they sound fine, but when I add them to a DVD, then preview it, you cannot hear the sound. When I burned the DVD and played it on a DVD player, you still could not hear the sound. What do I do? Help please.
You need to extract the audio from the video file with an audio program (I use cool edit pro 2.0 to do this). Save the audio as a raw WAV file (PCM format).
Next, you use the video for the visual and the new audio to create your DVD MPEG-2 file for DVD burning.
The entire process above is called de-muxing.
Get a copy of DVD-Lab. It de-muxes for you and creates the DVD files for you too, with menus!