It's because DVD's are deliberately crippled audio wise, they record them to -12dB instead of 0dB.
I assume since Microsoft software is so much better than that free linux rubbish, that you can just switch in one of your audio filters as an amplifier like -vf volume=12
Look it up in the WMP manual, I'm sure it'll be there.
Or, in the real world, use powerdvd and tick "loud environment" in it's options, however this does add compression.