this is how you block background sounds: put this in your userContent.css file:

/* block embedded sounds */
object[data*=".mid"],embed[src*=".mid"] { display: none !important; }
object[data*=".mp2"],embed[src*=".mp2"] { display: none !important; }
object[data*=".mp3"],embed[src*=".mp3"] { display: none !important; }
object[data*=".mp4"],embed[src*=".mp4"] { display: none !important; }
object[data*=".wav"],embed[src*=".wav"] { display: none !important; }
object[data*=".wma"],embed[src*=".wma"] { display: none !important; }

CaptainKirk @ A. Bersvendsen

Had to look for a way to do this after having exposed my ears to the front page of Finland-Korea Friendship Association, while looking for comments on North Korea’s latest feat.

Well, it doesn’t seem to help. The horrible jingle is still played.

No, wait – it’s just those stupid fancy quotation marks WordPress has shoved into CK’s comment – so replace any occurences of “ or ” in the above with a regular, plain, sideless double quotation mark, a ".