Manual is there for a reason. It is the button to the left of the volume switch.
AUTO u (Automatic Volume): With automatic volume, the audio system adjusts automatically to make up for road and wind noise as you drive.
Set the volume at the desired level. Press this button to select LOW, MEDIUM, or HIGH. AUTO VOL LOW, AUTO VOL MEDIUM, or AUTO VOL HIGH displays. Each higher setting allows for more volume compensation at faster vehicle speeds. As you drive, automatic volume increases the volume, as necessary, to overcome noise at any speed. The volume level should always sound the same to you as you drive. AUTO VOL NONE displays if the radio cannot determine the vehicle speed or if the engine is not running. To turn automatic volume off, press this button until AUTO VOL OFF displays.
AUTO u (Automatic Volume): With automatic volume, the audio system adjusts automatically to make up for road and wind noise as you drive.
Set the volume at the desired level. Press this button to select LOW, MEDIUM, or HIGH. AUTO VOL LOW, AUTO VOL MEDIUM, or AUTO VOL HIGH displays. Each higher setting allows for more volume compensation at faster vehicle speeds. As you drive, automatic volume increases the volume, as necessary, to overcome noise at any speed. The volume level should always sound the same to you as you drive. AUTO VOL NONE displays if the radio cannot determine the vehicle speed or if the engine is not running. To turn automatic volume off, press this button until AUTO VOL OFF displays.