Checking the dxdiags posted, what you could try is to update the sound drivers. I notice that they are from 2009 and updating them to the latest ones might help solving the problem.
Are other in-game sounds working correctly, and what if you 'fiddle' around with the audio sliders in the options menu? And are they back after a game restart?
The music works, and selecting armies sometimes seems fine, as does the effects when besieging a province. The biggest culprit are sounds such as the effect when clicking on a province, messing with the volume sliders doesn't seem to have any effect. They are generally working fine after the game is restarted but will mute after a short period.
Last edited: