Is the Audio stream an AC3 sound?
If yes, you can go to right-click -> Options -> Filters -> Advanced and then change the Voice's Gain settings for AC3 Filter (if you have installed it with KL MCP) this way the non-AC3 sounds will not be distorted and you'll be able to still listen to the voices over the music background.
If the problem is with movies with different audio streams (not only AC3) you can use BSplayer's Equalizer to rise the central bands (the ones in which the voice has its frequencies). I suggest you to raise bands from 600Hz to 3kHz (but it depends from movie to movie). Don't boost the audio too much or you'll have a great distorsion and the result will be really ugly. If you need to boost a lot the voice over the music, then raise the voice's bands, and lower the Master band (and/or lower the other frequencies bands)
