Ah, thanks. Good to know I'm not alone. Perhaps this will get fixed in the upcoming patch...but then maybe not, because it doesn't sound like a game-breaking problem.
If it helps, I have the problem too.
My recommended solution until Bethesda fixes it: turn on loudness equalization, http://static.reviewmylife.co.uk/2010/1112/loudness-equalization.jpg
I made a thread earlier about low sound and a poll. Of about over 200 people that voted, around 70% or so had low sound problem one way or another. That's big numbers. It's a big problem.
Gstaff contacted me earlier about it, so Bethesda is definitely aware of that the problem exists. I was surprised though to see this fix not on the changelog for 1.4 patch.
It's pretty fundamental to have properly working sound, even though it's not game breaking. That's like 1/4 of the whole game.
In general, it seems Skyrim has low sound for a lot of onboard stereo users.