Problem with sound?

I have had a working version of xbmcbuntu running fine off a hard disk and all is good.

However, this hard disk i want to use for storage as I was just using it temporarily to test. So, I installed xbmcbuntu onto a USB stick to free up the hard disk.

I then copied the userdata and addons folders over from the hard disk to the USB install to replicate the settings over.

All worked apart from now I have a weird problem with the sound. I can hear the menu noises but when trying to play music or a film I get a message "failed to initialize audio device"?

I have checked the system sound settings (also tried changing the settings too) but nothing seems to resolve it and the sound settings in xbmc do match what I had set on the hard disk install?

I am using the HDMI port on my graphics card for video and sound which is plugged into my receiver.

Can't make out why it works in one install and not in another?

Any help much appreciated.