Some of my computers have sound, my main one does.
I have not seen a general statistic, however.
Perhaps you could derive the number you need, or a lower bound on it, from the number of Skype users, or the number of WinAMP installs ?
If you need a much more specific statistic, such as the number of users of your web site that have sound installed and set up to play through the browser, perhaps there is a way to measure that directly. For example, you could pick a sacrificial group of users, and expose them to the most awful sound you can come with -- chalk screeches and cats through band saws and "Hey everybody I'm jacking to child porn!" and animal yelps and human moaning and stuff like that. Then you could compare how many of them never come back, to a control group that was not assaulted.
