go to dolby.com and download the spec sheets for 5.1 delivery
in this, standards (which are followed by most studios) are recomended (as tech spec comparisons) and advised within the thresholds of the Dolby Standards themselves)
If you dont follow the bracketed standards of what is offered within the encoder and information from these white papers, then you might be heading for trouble.

Do not forgt that the DD technology was designed for a wider DR, not a wider loudness peak level. By turning off your DR compression, you are potentially putting your audio at risk of peaking, excessive bleedthrough, innacurate mixdown and basically an unstable encode.
It could also put you in a position of non compliance to the DD standard.
I say potential, not actual as i havent seen the file to analyse it myself

In addition, it might jstu be that the theatre itself has increase the volume level of the centre channel, or running a seperate preamp for that channel. As i dont knwo the config, i cant say, but it seems that if it sounds ok everyowhere else BUT here, then more than likely that is the problem

There could be a more fundamental problem, well if this cinema is anything like the ones I've shown movies in.
The standard Dolby cinema decoder unit only does surround from the film reader, just a bit stream from an optical reader on the projector. Another reader reads the stereo optical track, that's used in case the surround data track gets too high an error count. The decoder unti has a couple of extra "User" inputs and are labelled as User 1, User 2 on the back of the unit. Just a pair of RCA connectors. In the cinemas I've been in wired to a mididisk player. I'd unplug that minidisk from the unit and hook my DSR-11 in there, select User 1 on the decoder and I was set to go after a quick level check.
Now those user inputs are programmable and from what went wrong one day they can be set to decode Prologic, loose one channel and things got really funky!

Now all of the above might be totally irrelevant, maybe this cinema has home grown their own setup. As suggested above take along a commercial DVD with a good surround track, listen to it. Then check how the DVD player is cabled into the rest of the system. I think if they're running SPDIF into the decoder you should be past obstacle 1, if it's just a stereo pair you need to do some more work. Someone from here, maybe on the audio forum had worked out a backdoor way to get a Prologic matrix encode without spending money.

Should mention in the cinemas I've been in there's no readily accessable individual channel gain controls from memory, just a Big Knob that goes to 100, well apart from trim pots on the amps and them you don't touch.

Hope this helps and let us know how you go.

Bob.

Sounds to me like you may have your highs (treble) and/or your lows (bass) incorrectly set. The voice channel (center speaker) SHOULD operate within a fairly narrow mid band positioning... no real emphasis on highs or lows at all. Depending on the original soundtrack, you MAY actually have to CUT BACK on the highs/lows

Don't know how this "theater" you describe is set up, but some of them actually do a bit of mixing of high/lows (in particular the highs) on different channels to get that "wide" feeling to it. If you add similar highs from different channels that have been "normalized" (or in other words brought as close to 0db as possible) then the addition of the signals will boost the highs over 0db thereby producing distortion.

If I were you, I would run off a test strip of the voice audio with the highs/lows EQ'ed way down and see if that does not clear the problem.

Thanks everyone for the quick and detailed responses!

I admit I am nearly clueless about audio, so I'm still confused...

I normally set AC3 dialog normalization to -31 as suggested, and the Modes to "None".

Then, in searching for solutions to my current issue, I read the following thread:

http://www.sonycreativesoftware.com/forums/ShowMessage.asp?ForumID=4&MessageID=496592

the highlights of which are:

from SimonW:
"Actually, one of the best ways to make sure that the dialogue is not too quiet is to first of all render the dialogue channel sound out into a WAV file.

Then load it into Sound Forge and go to the Normalise dialogue. Select Average RMS, and then go to Analyse. It will then tell you the average volume of the sound.

Now when you render out the AC3 file from Vegas, set the dialogue normalisation to the closest setting to the average result you saw in Sound Forge.

"

and Bob (farss):

"Technically what Simon is saying is correct, you should do that as well as setting the preprocessing to None. If you don't have a way to separately process the dialogue then use SF to just read the RMS value of your soundtrack and use that as the dial norm value in SF.
If you don't have SF or if your work is not for theatrical release, what the heck, set it to -31dB.

"

and again simon:

"I would recommend against blindly setting it to -31. Certainly for good sound mixing.

The reason that Dolby include this dialogue normalisation option is to make sure that the sound plays correctly in relation to reference level no matter which DD equipment you are playing it on. But this can only work properly if it is set to the correct setting."

BUT TGS above reiterates using the -31 setting.

I have limited access to the theater so I'd like to prepare some burns ahead of time for testing. I'll go back and make another clean burn using the -31 setting. I have the burn created using the above method recommended by simonw (actually I used the method recommended by farss, since I don't have separate audio / music tracks available to me.)

Any other suggestions for a trial burn?

When I next get access to the theater, I'll see if I can get into the booth to get a look at the equipment hookups. (not allowed in my previous visit, alas.)

Is this a theatre or a cinema?

Really until you know what's going on and how things are hooked up I would avoid futzing with your encoding, the issue could have nothing to do with it at all.

Bob.

It is a cinema. "Studio Movie Grill". They show 1st, 2nd run theatrical releases, but also rent the theaters out to groups for meetings, special showings, etc. They are equipped to show DVD productions.

Although I have used the -31 setting in the past, I hate "HOT" signals so much, that I backed off and now set it to -29. That is just a precaution. The -31 didn't really sound hot, but I also use Sony Dig 8s and the built in mics on those always sound like they're bordering on distortion. Personally, I don't need the loudest signal. I just need to be close.
I do this because if I hear something bordering on 'Hot', I now assume it is the mic on my camera and not my ac3.
One other thing you can check from the timeline before making an ac3 is that your levels are set for a clean, undistorted signal. I'm not really sure if you said you were using 5.1 or Stereo. But the "Master" volumes of the main output mixer should be set to 0.0dB and the individual track volumes or track volume envelopes should be used to get the levels balanced so that in the Master volume VU meters , the levels are in the right neighborhood. Once that is done, you can use the Masters volume controls to make changes in the overall mix level.
If your current output mixer Master Levels in Vegas is set to -6dB. or lower, you may be mixing in some slight distortion. It means the sum total of all your track volumes were so loud, you had to back off the main mixer the get them back down. You get a cleaner signal if you mix with the individual track volume or envelope and leave the Master out mixer set at 0.0dB. Once you get the levels right that way, then changes to the Master volume are safe to make.
Don't forget to take a regular released DVD with you with levels that are very close to your DVD. (or close to one of the DVDs you take)