It's specifically Google Navigation. It does it for my Captivate as well as for two of my friends who have them. You have to select a destination to navigate to, and as soon as it goes into 3d mode, the system volume gets set to max.
Everyone else I know who has an Android phone is out this week, so I can't say if it's a Captivate thing, a Galaxy S thing, or an Android thing... Can anyone else see if the same thing happens on other platforms?