I have the exact same issue too, it is very disturbing. I have an Asus Xonar D2 if that matters.
I have a Xonar DX and have the same problem, the whole sound has a strange echo, even if I disable every filter in my soundcard, 3d effects, >2 channels, etc.
Tested with headphones with only stereo and everything disabled (Dolby, etc) and it's the same.
I have done a little workaround for myself (I'm using headphones), maybe it works for others too. This is how it works:
1. Open up your sound card driver menu (in my case it is the Xonar D2 Audio Center)
2. Make sure the audio channels are set to 8 before you start Divinity: Original Sin
3. Start Divinity: Original SIn
4. Load your savegame or better, hit Continue
5. Switch back to your sound card driver menu
6. Set the audio channels to 2, then switch back to DOS
7. The sound volume of DOS now should be way lower than before, so turn it up yourself ingame
This should reduce the echoes. Don't forget to switch the audio channels back to your preferred number when you're finished playing DOS!
If this doesn't help I'm clueless, sorry
