Typically in a multichannel sound set-up, the dialog is passed through the centre channel. There are a few options that you can use.
First, in ffdshow audio decoder or AC3filter, or whichever filter you're using for sound (since you mentioned AC3filter, I'll use that example):
Go to the 'Mixer' tab and increase the Gain on 'Voice'. This will boost dialog, so you don't have to increase the volume to hear it. It leaves the bass and surround channels untouched.
OR:
In the 'Main' (default) tab, enable the 'DRC' checkbox. This stands for Dynamic Range Compression. What this does is decreases the playback volume range. That means, in general, quieter sounds are played more loudly, and louder sounds are played more quietly. Therefore the difference between the quietest whisper and the loudest explosion is less dramatic than the default in the movie file. This feature is also known as 'night mode' or similar in some AV receivers.