do you un tick all the audio boxes on the right in shrink and just keep the AC3...which is 5.1...that'll only keep the 5.1 on the movie only
some of the extras are not always in Ac3 5.1,if your going to keep extras, then you need to be careful when you deselect audio streams. When your previewing extras, you can right click on the preview screen, while your chosen extra is playing, to find out what audio stream you need for that extra but for the main movie,yes you usually want the 5.1