Take a video with sound, e.g. c:File:Protesters fighting against OMON police with snowballs.ogv. Upon loading the page, right of the loudspeaker icon there are three bars, indicating 100% volume. When I click on the icon, the bars disappear, indicating 0% volume. At this point I expect no sound when I click play. But when I do, the three volume bars appear again and sound is played at full volume.
Work-around: when, after loading, I drag the volume bars to zero, no sound is played as expected.