When I play DVDs, I have trouble hearing. It sounds like everyone is mumbling.
You didn’t say if increasing the volume while playing the DVD clears up the mumbling…
I don’t know what causes your particular issue but I have long has “issues” with the volume of my TV. The primary amplifier for the Television simply plays through whatever sound is being sent to it by the TV or DVD. The result is some TV programs are too quiet, some are too loud and almost all commercials are insanely loud. The DVD sound varies from too quiet to hear to loud enough to give you headaches.
I searched Google for an answer to this ridiculous shortcoming in technology a few years ago and came up with a Constant Volume Amplifier. The one that I am currently using is Audiovox Terk VR1 Automatic TV Volume Controller currently on amazon.com for about $21.
All of your media components hook up to this thing via RCA jacks and you can set the volume on your components at mid range, then set your Volume Controller at whatever level you are comfortable with and the volume level will not change.
This has solved for me the issue with my DVDs being too quiets in some scenes and too loud in others.