I would guess something else is happening in your computer that interferes with both the sound and video. Quite what I'm not sure!
You don't get tis in EU4 I assume?
Does it affect music, sound effects and ambient sounds?
What if you play music outside the game, in Spotify or WMP, does that also get affected?
DXDIAG is a program you run from a command prompt or the Windows start menu 'run' dialog box (or 'search programs' in Windows 7 or later). After running it will open a window and start collecting info with a progress bar in the lower-left corner. When it completes click the 'save all information' button and save it to a file then attach that file here.
Please attach your Documents/Paradox/Imperator/pdx_settings.txt.
From your Documents/Paradox/Imperator/logs/ folder, attach system.log .