Oh crap! I totally forgot to wish my Dreamies a happy B-day! Time to fire up some MvC2!
The VMUs make that beep when the battery inside is dead, and the controllers will drain the batteries if you leave the VMUs in when the system is off. If you take your VMUs out of the controllers you won't have that beep when you power on your DC. If only I had known that before it was too late.
Huh, that's helpful! I'll do that from now on, even though my VMUs, all six of them, are probably dead right now. :/