I'd watch that one.
This smells like an older question. Read link.
So that's what I'd do next. See if a new account behaves.
After upgrading from 10.5.x to 10.6.8, every time I boot up my iMac 2.4 Ghz Inte Core 2 Duo, the System Preferences Sound OUTPUT changes from Internal Speakers to my external mixer (Tapco-Link.usb)!!! HELP!!!!