I don't think you can adjust it from the steering wheel, because the phone's audio system is doing the mixing and the car radio is just playing the summed output from your phone. You may be able to adjust the spoken volume by pressing the + or - buttons on the side of the phone while the directions are speaking. If this doesn't work, I don't think there is a way to adjust the volumes separately. Depending on what the music comes from, there may be a volume adjustment slider there so you can turn down the music and leave the nav instructions at their normal volume.
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