You can use BSplayer's equalizer (Atl+G), or a Winamp DSP plugin (I suggest you enhancer).
Probably the low audio is related to the codec used to encode the movie. For example, AC3 audio usually is low, but you can raise the valume level in AC3 Filter preferences, so you can use the same volume level in BSplayer for all your movies and at the same time to hear the conversations on your AC3 encoded films. I don't know if AAC have such a problem and a similar solution, but maybe you'd better take a look at AAC filter preferences (I think you'll find them in
START -> All Programs -> K-Lite codec pack -> Configuration)
