No audio when using Windows Media Player for streaming

Guess most of my readers would be familiar with this situation - every once in a while you are called to give tech support to your parent, because something isn't working with their computer.

Two weeks ago the graphics card in their computer, based on GeForce 6600, passed away to a better world, so I got them a new one. Ever since, every problem they have is immediately connected to the graphics card, no matter it has nothing to do with graphics.

The latest problem was quite a challenge - there was no audio when streaming (audio and video) from web sites. The problem didn't reproduce in different accounts, or when using VLC instead of Windows Media Player. Since nothing special was found in the Media Player settings, I had a feeling the answer would be in the registry. Here is the solution. Three things surprised me in this solution:

1. How comes it affected only one account? The registry key is not user specific.
2. What had changed? The value of this key was the original value which is the default. So this means there's another reason of this problem, but it's nowhere to be found.
3. How did the guy that found the solution did that? Changing such an obscure registry key.

Well, the problem is solved now, and I convinced my parents it has nothing to do with the new graphics card. This is definitely the hardest type of tech-support, as you must be nice and must give full details. Also, you personally care about the users.