It is possible that you have a flaky event with the nav OS. I would try downgrading to an earlier version of the nav OS, then re-upgrade to a known good, original copy of the latest release CD. Then see if this has any affect on the nav volume behavior.

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SOLD
2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input