I have this problem as well... You're watching a movie at night or something and you have to turn it up just to understand what people are saying, even then it's sort of difficult,, then the scene changes and their is music pumping and you realise "fuck that's loud!" and have to turn it right down, then you can't hear it again.

A part of this is just that the music is a lot louder than the talking in the actual cut of the movie itself, so that is the sounds/director's fault.. Not that it was an accident or anything, it's probably done on purpose, but it's not your t.v or dvd players fault...

I have the same thing when i'm watching free to air tv. The movies are kind of low volume, but the advertising f'n screams at you and I ALWAYS have to turn the television down about 30% when it goes from a quiet movie to advertisements..

A part of the problem also is that a good stereo system will be proficient and producing all the sound frequency range at an equal level. Talking will have it's full resonance, strength & clarity, where as normal television speakers and some stereo speakers don't really produce voice THAT clearly.. So you generally have to turn it up to hear what is being said clearly. But just about any speaker can blurt out "noisy" music pretty well.. Again it won't be super clear or anything, but it'll scream compared to the unclear voices...
The only real solution is have a really good sound system that reproduces the sound really well. You don't get that high/low effect from the sound.. Occassionaly it's louder and quiter, but as everything sounds so natural, it doesn't just sound like someone has cranked the volume up all of a sudden...
Also, if you watch a lot of dvd's, t.v. or play games, you should invest in a good pair of headphones and a headphone amplifier that will easily plug into the output of your dvd or t.v.... You'd have to spend about $4000 - $5000 on an actual stereo system to get the same clarity and sound spectrum that you get from a decent $400 pair of headphones and $300 amp...
About 2 years ago I spent $800 on a heaphone and amp combo and they are very rarely off my head while at home on the p.c. in front of the t.v.... They are just freakin awesome and you hear everything from footsteps to russeling of the actors clothes to jingle of the change in their pockets.. The sword fights in Pirates are amazing! You can tell if their blades are sharpened or slightly jaggers by how the sounds against each other...
It's freakin stunning and you don't have to worry about the up and down volume as it all sounds freakin amazing and doesn't cost to much...

Here's a link for the heaphones I bought : Sennheiser HD600
Sennheiser Worldwide: HD 600 - Headsets / Headphones Professional - Equipment Manufacturer - Information,Test


and a link to the amplifier I bought... It has an inbuilt dac and USB connection, so you can plug it straight into the USB of your pc, and it detects it as a dac, and completley bipasses your computers internal soundcard and just uses the amplifier to convert the digital signal straight to your headphones... The amplifier just makes everything sounds completely stunning through the headphones.. The higher the volume, the more clear and crisp everything sounds... They seriously hurt my ears before the sound starts to deteriorate in any way.. The amp and heaphones are my 2 favourite objects in my whole entire house...
The below amp is basically the same as mine.. They changed the name and put some weird circle thing on top of the case, but otherwise exact same.
Meier-Audio - Corda Aria Headphone Amplifier
Meier Audio: Corda Cantate | Headphonic: Australian Headphone Specialists: Buy Etymotic, Alessandro, Audio Technica, Ultimate Ears, Talisman, Meier Audio and more