The ‘cue’ family of properties allows the attachment of key sounds (or “Auditory Icons” as the CSS spec refers to them) to elements. This allows an author to add distinguishing characteristics to semantic elements. This property specifically references a sound to be played before the element has been aurally rendered. If loading of a URL fails, nothing should be played, but if the system does not have the ability to play it, the spec says the system should try to use some sort of cue (audio, visual or other.)
<span style="cue-before: url(sound.wav)">text</strong>
inherit: Explicitly sets the value of this property to that of the parent.
none: Play no sound before the element.
[URL]: Indicates the URL of a sound file to be played before the element. If loading of the indicated URL fails no sound will be played. If the referenced resource is not a sound file, it should be ignored and the property should be treated as if the value were set to ‘none.’