In the future they should add a ambient sound volume slider or tie it into one of the other sound categories.

For now I recommend leaving all volume at near max in the in game configuration setting but using Windows 7 or Windows Vista's Audio Mixer to place XIV at a volume that is comfortable since adjusting the volume in game will only result in the music, sound effects and the menu sounds getting quieter while the ambient sounds remain the same volume.

I usually find about 20% is reasonable personally.