Some videos have various audio options. Some are not required. For music vid's, if you have the choice b/w pcm (= aiff) & ac3 then ac3 will take up much less space ... but just make sure that taking one out doesn't affect some particular section of the vid. The sample below is not pcm but mpeg audio but was the only example I could find handy at my fingertips. (as a side-note mpeg audio is only available on pal dvd. yipee!)

I almost always encode using ac3 audio so I can maximise the space available for video. I create lots of videos from full quality video.

Lord of the Rings

Now I see how you know what type of audio is on a dvd, can't tell you how many times I have seen that and clicked on one, but not ever thought about the different types of sound !!! How dumb is that ?
The way I look at what I am doing is the more I learn the more things I will try and eventually I will find myself a little nitch. I have to get to that stage. I envy you because you have got to that stage.
Information is power!
Thanks, this particular screen shot was really good to see.

Ciao Merelyn