Originally reported here: https://bugs.launchpad.net/nuvola-player/+bug/1049253
WebKitGtkLauncher starts with sound muted (at the system level) and doesn't reflect changes to volume made within the WebKitGtkLauncher. Specifically for website http://www.rdio.com. Once unmuted (through manual adjustment in system volume) sound works as expected i.e. changes made in WebKitGtkLauncher are reflected in system sound levels.
Video of the problem.