Horrible Sound in Bootcamp
If you haven't done it yet, first check the system on the Mac side to make sure sound is normal. (great point bmike) You also don't have extra software on the Mac side modifying the default sound, do you? Like in iTunes how you can adjust the EQ settings?
Others have commented how Apple is not spending much effort adding Equalization optimizations to BootCamp. Apparently there were issues with speakers blowing out so they also adjusted the volume levels.
My recommendation is if it sounds like the issue is only occurring in Bootcamp, then you may want to provide Apple some feedback:
https://www.apple.com/feedback/bootcamp.html
Here were additional
Comments regarding Windows 10 BootCamp sound differences on Reddit