Hi - I had a similar problem with my X3 with icelink. I could set Nav Volume for radio or CD to ideal level but when I switched to the ipod source the nav volume would suddenly increase to shouting level !

The NAVMIN and nav volume setting appear to be common settings that cannot be adjusted independently by source - despite being available from different menu's. The NAVMIN also is used for both RDS broadcasts and NAV voice commands to ensure reasonable volume when radio or other source volume is turned down to zero or muted.

I fixed the ipod problem by reducing the input voltage of the icelink down to 1v - this seemed to be closer to the radio volume and evened things out.

You may find that your sources are pushing too high a signal into the nav causing the nav voice to over-compensate ?