playing the targets of cue and play-during

whose responsibility is it to fire and/or play the targets defined 
with the cue-before, cue-after, cue and play-during properties? 
[note 1]

the UserAgent's native audio support, the underlying OperatingSystem 
which provides system sounds (earcons), or the assistive technology?

i am willing to concede that it is the assistive technology that 
must trigger the aural rendering mechanism, but it should be a 
backplane event, like system sounds in the windows environment, or 
the annoying embedded sound in a web document set to play in a loop

Note 1:
   * cue-before and cue-after in CSS2
     <http://www.w3.org/TR/css2/aural.html#cue-props>

   * play-during in CSS2:
     <http://www.w3.org/TR/css2/aural.html#mixing-props>

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ACCOUNTABILITY, n. The mother of caution.
                 -- Ambrose Bierce, The Devil's Dictionary
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         Gregory J. Rosmaita, oedipus@hicom.net
      Camera Obscura: http://www.hicom.net/~oedipus/
UBATS-United Advocates for Talking Signs: http://ubats.org
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