[css3-speech] cues, volume control

Dear all,
I don't understand the rationale for this particular statement, in [1]:

"
Note that ‘voice-volume’ does not apply to audio cues for which there  
is a separate means to set loudness (absolute, or relative to ‘voice- 
volume’).
"

I totally agree with audio cues [2] adjusting their output levels  
relatively to the 'voice-volume', but why should they be able to set  
absolute volume values ?

For example, a text with "voice-volume:silent" should have its cues  
muted as well, yet audio cues can override this. By analogy to the  
visual 'visibility' property (and by virtue of the "aural box  
model" [3]), the borders of an invisible element are now shown: the  
entire element (content, padding, border and margin) remains involved  
in the layout, but is "transparent" (which is equivalent to "silent"  
in the aural dimension).

If nobody objects, I would like to restrict volume control for audio  
cues in relative terms only (percentage applicable to the voice-volume  
value).

Sorry if I am missing a point here.
Many thanks, Daniel

[1]
http://dev.w3.org/csswg/css3-speech/#voice-volume

[2]
http://dev.w3.org/csswg/css3-speech/#cue-props

[3]
http://dev.w3.org/csswg/css3-speech/#aural-model