I have the same problems with Borderlands GOTY .
Was watching a youtube "Let's Play" style video and the poster mentioned having sound issues at best quality sound setting. Bumping the sound down to the next best settting fixed the issue for him. Perhaps not the most desirable solution, but it made the game playable for him.
I've only found one solution that works. I've had to go into my Bios and turn on the onboard audio (turns out, that's Realtek High Definition Audio), then plug my speakers into the corresponding socket and disable my sound card.
Doing that allowed me to then go into the properties for that audio device (displayed as High Def speakers) and to finally select the Loudness Equalization option on the Enhancements tab.
That tab and the loudness equalization option are not available through my sound card. Hence the convoluted work around.
It's annoying, but at least I now have sound at an acceptable volume. The only other solution would have been to buy a 4.1+ speaker system. No easy task where I live =/.