For a more up to date post see this post
Stereo soundtracks are usually played with Dolby Prologic 1 or 2 decodings in cinemas. Many cinemas have not the option to use Dolby Prologic on stereo DCPs. If the stereo soundtrack is played in stereo, the audience not sitting in the middle will hear the dialogue coming from either the left or right side.
This makes a 5.1 mix essential on DCPs. Film festivals often specify that they do not accept stereo sound.
A good 5.1 sound mix is important, but if you do not have the budget you can use one of these 5.1 mixing plugins:
Pro Tools Ultimate plugins Penteo. DTS Neural™ Surround Collection
Pro Tools Ultimate has a fast bounce to disk mode.
Normalizing
-You often need to normalize the dialogue on the stereo soundtrack to a lower sound level before using the plugin to avoid clipping.
Limiters
-Limiters will often alter the relationship between dialogue/music/effects on the stereo mix. But sometimes you may need to lower the dynamics on stereo mixes.
Phase detection mode
Pro logic uses a phase detection algorithm to convert stereo to 5.1. Filmmakers are not always mixing stereo sound in Pro Logic so you can try other algorithms.
DTS Neural™ Surround Collection
-Uses a phase detection algorithm.
-I have used it on the Auto setting to convert different materials to 5.1.
-If the calibration of sound levels is OK this plugin works great on a lot of material.
link
Se this post for more info
Penteo Pro
-I have used this with both music and film soundtracks.
-The LCR mode is compatible with cinemas with bad sound calibration.
-Does not have a phase detection mode.
Link
I think in theatres with a pure digital setup, stereo DCPs can’t always be processed to a surround signal like Pro Logic does.The Dolby CP750 has no way of doing this for 2.0 DCPs by choice I believe, you either configure it to always do it, or not.
Which means if the patron sits on the first seat of row 5 in an auditorium with a fairly large screen, dialogue is not coming from the mouth that’s talking, but really from the side, which can be quite annoying, so I also suggest making at least a 3.0 mix (L, C, R) and fill the remaining channels with silence for a 5.1 DCP.
On a pure digital setup with Dolby cp650 with cat778 AES 8 channels in/out, I use Format 80 programmed on button 1 for stereo soundtracks and format 88 programmed on button 11 for 5.1.
On a Doremi you can see in the channel count in editor/playlist/item properties.
ALso the CPL title should have the channel count.