Lexus 2003 LS Owner's Manual page 491

Shaded text indicates operations that cannot be performed
while vehicle is in motion.
Voice Guidance Option
Voice Guidance Low
This preference is the lowest volume setting available when
operating the voice recognition system.
Voice guidance can be used only to control the volume level
of voice recognition when in operation. This option is not
used for volume level control during an actual call.
Voice Guidance Medium
This preference is the default setting in the system when first
activated in the vehicle. This is considered the standard level
of volume.
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Section 3
NOTICE
INFORMATION