Skyrim seems to have really bad sound support for different sound and speaker hardware. With the right hardware, the sound is beautiful. But with the wrong hardware, sound is, as you say, very low.
It's a very, very common problem and it's almost outrageous that Bethesda has and never will do anything about it. I have complained about this myself since basically the first week Skyrim was released, and Bethesda are aware of it. It's not only poor manners of Bethesda to do this to us customers, but it's also poor manners to Mark Lampert who has been creating all these sounds, which are otherwise really, really well done.
I have tried three different speakers with Skyrim. The first was a 2.1 Logitech, which had low sound. The second was 5.1 Z-906 Logitech speakers, which not only had low sound, but also a bug with the NPC voices (if I turned my head, you couldn't hear a word of what NPCs said). This was tested on the same computer, which is proof of that Skyrim has some serious compatibility problems with speakers.
I have now bought an entirely new computer with new high quality speakers. The sound card I now have is Creative Sound Blaster Z and the speakers are Audioengine A5+. The sound in Skyrim (and also everywhere else; the speakers are amazing) is now more beautiful than ever and I have no problem with low sound. The only problem that I now have is that magic sounds are way, way too loud, so I had to create my own sound patch so I don't go deaf.
So in short, if you have the wrong type of hardware... sorry man there's nothing you can do about it. Loudness Equalization is a very poor suggestion because not all sounds should be equally high (or low); if you're an audiophile this suggestion is downright a joke. The only real solution seems to have the right hardware that Skyrim supports. Which this hardware is is something no one seems to know. I first thought that 5.1 speakers would solve my problem, but as it turns out above, this was not the case. In very short: you're either lucky or unlucky.
Also, you could try this:http://skyrim.nexusmods.com/mods/33740/?
It's definitely worth a shot.