I don't know that Dolby Dynamic Range Control will work for anything other than Dolby Digital compressed formats (i.e AC3 and newer Dolby compressed data formats). The dynamic range compression is accomplished using meta data stored in the encoded audio data that tells the decoder software running on your player when the audio peaks or drops low. Other formats, such as raw pcm, or mp3 doesn't have this data (at least not any such data that can be interpreted properly by a Dolby decoder), so Dolby Dynamic Range Control will not have the information it needs to do the time varying volume adjustment.
Perhaps the sound card is trying to use Dolby Dynamic Range Control but feeding garbage data, so it is changing the volume randomly.
The soundcard should be checking to see whether or not Dolby Dynamic Range Control meta data is present in the audio data stream before it tries to apply DDRC, but sometimes the programmers mess up and forget to include code to handle certain scenarios.
Try playing some Dolby Digital material from a DVD, and turn dynamic range control on and off (night mode) to see if it works properly, then try the same thing for an mp3, or a raw wav file. If it works for the Dolby Digital stream but not for the others, then what I describes is indeed the case.

Dolby has a solution to the inconsistent volume problem on the way however. It's called Dolby Volume, and it does a great job of leveling out the volume of material that jumps up or down in volume (e.g. two ipod tracks that have grossly different average levels, or an annoyingly jump in volume when a tv commercial comes on).
It's very new, and I don't know that it's available for pc's yet. It's rolling out first in new tv's this year.

The video does have its own volume settings, but there is also the windows volume which is like a master control. The Windows volume will override the video volume. Never had problems with that.

The volume of a video can come with preset value, but once you change that using the master control Windows volume, the auto-volume for the video will change accordingly. You could probably also try to use file extension software to see what format is being used for the video. That could offer more information on the problem.