I'm not sure if I can ask this, so feel free to tell me to shut up.

I am a QT7 pro user. I just downloaded a few of the HD trailers and I think they have 5.1 sound, but i can't tell because my PC does not have 5.1. Just hypothetically, lets say that way back before we had QT7, that I used to take trailers and master them for DVD so I could watch them before films. Now lets say that I would split the audio from the video using TMPEGnc and then turn the audio into ac3 using AC3machine. Now lets say that I tried this with the new HD trailers and when I mastered the DVD there was no sound. So, just hypothetically, what would I be doing wrong?