Originally posted by: Arnie.d
For the stereo mix, if I want to recreate the audio on a 5.1 system as closely as it was heard in cinemas how would I do that. If I encode it as AC3 stereo in surround mode, would that be accurate? Yes, the idea is that those 2 channels contain matrixed surround information. Play the audio through a Pro-logic decoder and you will get 4 channel output. This is the same process used in theaters for Dolby Stereo tracks.

The format doesn't have to be AC3, PCM will work just as well. The only advantage in using AC3 is that you can set a flag in the stream to indicate that the audio contains Dolby Surround information. If this flag is present it normally turns on the Pro-logic processing in your decoder, saving you the trouble of pressing a button to select it manually. This kind of audio is denoted as DD 2.0/s or DD 2.0 surround (as opposed to plain stereo which is just DD 2.0).
Originally posted by: Arnie.d
For the mono mix, was there the same audio coming from all the speakers?

No, just one central speaker. This audio is presented as DD 1.0.