Had same issue with Windows Media Player, this fixed it:
Under the Now Playing tab is a small downward pointing arrow.
Click to access options.
Select More Options.
Click on the DVD tab.
Click Advanced.
Tick the Mixer radio box.
Voices were then louder in the mix.
I also selected Noise Reduction, just to get rid of some hiss.
I also selected Dolby Decoder, but noticed no difference through my earbuds plugged into my laptop.
Hope this helps, fixed it for me.