WebAudio specification https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/specification.html#AudioParam shows that computedValue should be part of the AudioParam IDL. It is not implemented yet.
@Chris: Referring to https://www.w3.org/Bugs/Public/show_bug.cgi?id=21545, It appears this node shall be removed from the specifications. Please confirm..
Yes, for now it has been taken out of the web audio spec, so we can close