Title says it all really; the WebKit API doesn't seem to provide any way to control the volume of the content it's rendering, specially media files. This is a bit troublesome as it means that an application cannot easily control the volume of WebKit content, meaning you can end up with an application that has muted all other sound sources but WebKit will continue to play at the system's volume. This issue appears to affect Mac OS X the most, I expect the issue does exist in some form on other platforms, but they are better able to control sound output at an application level. In any event, it would be nice to be able to better control the volume of sound produced by WebKit.
WebView has methods to get and set a media volume multiplier. I don't know why, but in the Mac port they are in WebViewPrivate.h: // Sets a master volume control for all media elements in the WebView. Valid values are 0..1. - (void)setMediaVolume:(float)volume; - (float)mediaVolume;