You can mute the volume in the OS, or rename the '..\Divinity - Original Sin\Data\Public\Main\Assets\Sound' folder to get rid of sound completely. I don't know if you can selectively move files out of that folder to just mute voices (renaming the Characters.bnk file didn't help in a quick test).
Try lowering the main system volume and in-game max all the sound sliders. That has removed the echo for a couple people with this issue.
One person had a problem with echoes and static using headphones (speakers were fine), which disappeared if they disabled V-sync in the video options.
Someone in the topic My sound is echo-y
said it helped if they changed the setting for the audio channels in Windows to 8, started the game, Alt-Tabbed out and changed the audio channels to 2 and switched back to the game.