Well if its not your sound drivers, have you checked your speakers? Even if it doesn't do it with anything else, also try some headphones (or different headphones). Also you might want to try just turning off the game music and sound effects individually to see if its just one or the other.
Edit: Oh if you are using any of the expansions, see if the original Sins has the issue as well. If not its a patch problem, if so it something weird with your computer.
