[css3-speech] cue volume

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

# The loudness of prerecorded audio cues can be adjusted relative to
# the volume level of synthetic speech, inherited value from the
# ‘voice-volume’ property.

Given that cues are prerecorded audio, which presumably has a volume
of its own (that can fluctuate during the audio clip), how is that
volume matched to the voice-volume?

~fantasai