http://www.videohelp.com/tools/Dynamic-Audio-Normalizer
http://muldersoft.com/docs/dyauno_readme.html
This new tool by a respected dev is unfortunately command line only so far. To understand why you might use it I'll need to digress for a moment.
Movies are still intended to be viewed in theaters 1st -- that's where the initial [hopefully] big bucks come from. Theaters have a sound system you cannot easily replicate at home. Audio has a huge dynamic range because there, in the theaters, it can -- loudness might range from a whisper to thundering sound you can feel through your spine. Those audio tracks are then re-used for things like Blu-Ray & DVD discs -- typical DVD & Blu-Ray players, and even the cable box adjust the dynamic range you hear, though often not as much as you might wish.
The high dynamic range of movie audio can be a bigger problem when you watch that movie on a PC or tablet etc. -- some software compensates, some apps don't, but listening to the movie is often less than ideal regardless. The typical way of trying to lower the dynamic range is to apply compression -- the loudest sounds have their volume lowered, which the quietest passages get a boost. It's more complicated than you might think at first, and it can mess up the sound.
Dynamic Audio Normalizer boosts the volume of the quiet parts only. It does not touch the loud parts at all, nor does it normalize in the traditional sense, which means boosting the volume of everything to a max or preset level.
If you're not used to working with video & audio tracks, relying on the usual video converter or ripper instead, you could still use this with a bit of work -- basically you'd process the audio separately, then add or replace the audio in the file you got out of the converter or ripper. For a DVD you'd probably get the DVD on your hard drive [minus DRM], use something like PgcDemux to get the separate audio track(s), then run the converter or ripper using the DVD on your hard drive as input.
Using PgcDemux is easy & should only take a few minutes, but from there things don't get complicated as much as there are a number of steps. You'd have to convert the [probably 5.1] AC3 to stereo .wav, run it through Dynamic Audio Normalizer, then convert that .wav file to whatever format you wanted to use, e.g. AAC.