The highest possible rate of data playing on a DVD is about 10 megabits per second. This includes all items associated with the video track, such as soundtracks and subtitles. In a simplified sense, a Dolby Digital 2.0 track takes about .18 of a megabit and a 5.1 track takes about .44 of a megabit. The sound rates are constant, but the video rates fluctuate. The number of megabits left after subtracting the rates for the audio and subtitles (which I understand to be negligible as far as "bit budget" goes) you get the highest possible video rate. But the video on the DVD only very occasionally reaches that limit. Actually, it seems to me the length of the video content on the DVD is a bigger factor than how many audio tracks there are. The numbers above indicate you can put five 2.0 tracks on something and you still could get a pretty good maximum bit rate for video. But if you have to encode 4 hours of video rather than 2, the levels won't approach that peak very often.