When I installed Windows Vista just after Christmas 2006, I noted that I needed revised audio drivers for the Creative SB24 device installed in my Dell 9150. Things have moved on since then, Creative have released further drivers…which you would think would just work out of the box. Well, they didn’t in my case. After a little digging around, I discovered this post, it pointed me in the right direction.
In a nutshell, I unchecked the Digital Output Only checkbox, as shown below. If you do uncheck this box, be sure to close and restart Windows Media Player in order to see (hear) if this worked for you.