The driver monitoring sound refactor introduced AudibleAlert.preAlert
for the driverDistracted1 pre-warning, but it was never added to the
alert volume controller's volume_map. Unmapped sounds fall through to
the 1.01 auto-volume sentinel, so setting Distracted Driver volume to
Muted silenced promptDistracted but the pre-warning kept playing at
auto volume.
preAlert is only used by driverDistracted1, so it follows
PromptDistractedVolume like the rest of the distraction alert chain.