Wierd nForce2 bootup problem

When I boot up and into Windows, the sound is extremely muted but still audible - the only way to fix this is to open up nMixer and select/deselect the "night mode" setting - this makes it regain full volume.

Anyone have any ideas

a) why
b) how I can stop it.

Did you try to uninstall run driver cleaner and reinstall the drivers? When did you begin to notice this, maybe you could use the restore funtion to go back to when this wasn't happening.