Everyone is mumbling

Need some info first:

What model Onkyo? How did you go about setting it up? Did you use the internal setup in the receiver? Does it show a full complement of speakers? What are the channel levels, especially the Center? Are you watching TV or DVD? What surround mode shows up on the front display or in the OSD for either input?

Unless something is truly amiss, simply try raising your Center channel level to see if it helps.

You might consider picking up a calibration DVD like Digital Video Essentials, along with an inexpensive Radio Shack sound-level meter. You'll get better results than doing it by ear.

4 speakers and a sub? That's 4.1. Do you have a centre speaker? If not, turning up the centre channel won't have any effect.

Can I buy just this one disc or do I have to buy the entire package

Yes it will - as long as the receiver is told there is no center speaker, raising the center channel volume will add an equal amount of center channel sound to the mix for both the front left and right speakers.

But if the receiver thinks there is a center speaker, and one is not connected, all of the center channel sound (which includes much of the dialog) will be missing.

A cheaper alternative would be to use the THX Optimizer material that is on many DVDs. See the THX Optimizer Search page to see a list of movies with this feature on it.

(You would still need the sound meter)

Not on any receiver that I have ever seen. My center channel volume level is blanked out when the center is set to OFF.

An even cheaper and more accurate alternative is to simply use the receiver's built in test tones plus an SPL meter. Fine tune with your ears to confirm your settings.

DD and DTS movies are indeed very loud during loud passages. That is just the way that it is.

Why are you in the NEO:6 mode? You don't have that speaker setup, so it is a waste of time.

Also, when a DD (Dolby Digital) signal is downmixed (AKA center set to OFF), DRC is applied to the DD signal in most receivers. This makes those loud sounds sound quite a bit lower in level.

DTS does not use DRC during downmixing (or at any other time), so the loud sounds are loud all of the time.

Use DD only if the loud sounds are too loud for your listening conditions. If you use a full set of speakers, set your DRC to the softer setting.

Glad it helps. I would still recommend a thorough calibration using test discs like AVIA or DVE. The THX Optimizer is decent enough but delivers ballpark results in my experience.

Good luck.

Calibrate.