I found lots of complaints about this on the Internet (Vizio forum and others). I found this answer on one forum. I believe it is similar to what videobruce is saying.
"Unless you're experience a big difference between DVD content and OTA HD content, the volume level of you HD content *SHOULD* be lower than that SD material.
I wish I could find the post over on www.doom9.org
that elaborates on the specifics, but AC3 content is mastered in a completly different manner than SD audio signals, or even music to matter.
To allow for the increased headroom and dynamic range that AC3 provides, on top ensuring that activating the various supported processing modes (Theater, Midnight) don't blow you're speakers during that Sauron Death Scene in LotR:FotR , AC3 audio is typically mastered as an RMS level or profile of ~-24db (if done right). An SD broadcast on the other hand, is highly compressed from a dynamics perspective and may go as high as ~-6db RMS, especially on commercials. Given that the scale is exponentual in nature, that equates to quite a difference between the two audio sources.
In my case, an MCE volume of 20 for SD content just about equals that of HD content (which I can't set the volume on since I'm doing SP/DIF passthru). "http://thegreenbutton.com/forums/thread/47068.aspx
"The sound volume difference between PCM and Dolby Digital original material has always been like that. Most HD programming has DD original sound. It's the nature of the two systems. Dolby says it's to allow a wider dynamic range. Anyhow, everybody else has the same issue and I've never seen any way to really effectively make a difference except to expect it and have the remote in hand. It's especially aggravating between programming recorded in DD and commercials recorded in PCM stereo."http://forums.directv.com/pe/action/...ostID=10288191
However, a TV broadcast engineer emailed me the following:
OK, then it is the DVD recorder. There's no external inputs involved, and the DVD recorder ought to be internally matching up the levels between it's analog and digital receivers. I still disagree, though, that there's any overall standard that says "digital TV is quieter than analog TV". If it's happening, it has to be a function of the receiver. But, I know none of my receivers exhibit that (and I have a couple brands and models at home and at work: RCA, Samsung, and DirecTV and Dish Network boxes that receive both over-the-air HD and HD via satellite).
"But maybe there are receivers out there that are consistently running all digital stations quieter. Since I have not encountered one personally, I can only theorize that those manufacturers are not setting the nominal level of their digital tuner outputs to a -20dBfs reference. Perhaps they're setting their DTV receiver output stages to a full-scale 0dBfs reference like you would with a DVD/CD player. But, that's just wrong-headed if they do. DVDs and CDs go through a post-production mastering process in which their audio levels are "normalized" up to maximum scale. They can analyze the entire recording, find the maximum audio level peak(s), and set it so that those peaks run right up to 0dBfs. Television broadcasters, and other industries that work with "live" audio will never do that, they're working with live incoming audio streams that they cannot entirely control. They have to leave some headroom, otherwise unexpected peaks in the live audio stream will overdrive and distort the audio. Thus the -20dB standard used in "professional" audio. Consumer DTV tuners should be following that same standard, and when they integrate those tuners into the same box with analog tuners, DVD/CD, and DVRs, they need to make sure the varying audio reference levels used in those technologies get lined up evenly at the output stage.
Well, thanks for the interesting information on your receiver."