Comment 4 for bug 305828

The problem involved having TWO sound cards, one on the motherboard and a PCI. Although the PCI card was set as default, apparently system sounds were designed to default to the motherboard card if available. the conflict between the cards was, apparently, shutting down both.
I discovered the solution to the problem in the answer to another question, where it was suggested blacklisting the motherboard card was the only way to shut it down completely. I did that, with immediate favourable results!
I apologise for not posting the solution. I realize it could be of help to others!