html5 video not playing audio / muted

my html5 video starts playing (in Chrome and Safari) without audio and mute button seems disabled. so i'm unable to un-mute. the player has sound in the file, so it plays properly in a player (example vlc). check screen below

<video class="media" width="920" height="520" preload="auto" poster="img/posters/poster.jpg"> <source src="video/desktop/resized/film.mp4" type="video/mp4"> Your browser does not support the video tag. </video>

any ideas?