Cannot change default audio device in Vista64

I have 3 soundcards installed on my computer.  All of them are working well - I can for example change the output from Winamp to any of them and get the sound.  The problem is that I can't change the default soundcard for the entire system.  This is obviously a bug in Vista 64 - I set the 2nd or 3rd audio device to be the default, a green tick mark appears next to the selected output, I click OK and...  nothing changes.  When I open the list of sound devices again, the first soundcard is still selected (in my case USB headphones).  Grrrr....

This drives me crazy.  I built a new computer, spent 10 days tweaking Vista (changing it from a system for dummies into something usable) and now I discover that it all has been a waste of effort because of this stupid little bug.   I have searched the internet for a solution, but I have only found dozens of other people having similar problems.

In Windows XP I could change the default audio output by editing the registry, but in Vista the audio stack is completely rewritten and I have no idea where this information is saved.

Help!!!