Problems of that sort are unlikely to be html-related; that was a general point about testing web-pages. For what it's worth, I don't see anything wrong with your coding, though you might like to know that the horizontal rule colour definition appears to work only in IE, and users of most other browsers will see a default grey instead of the red you intended. I can't speak for Mac browsers.

Sometimes, when a browser fails to open a media file, it is because the browser doesn't have a built-in component that can handle that particular format, or because a third-party plug-in is required for it. Installing the appropriate plug-in will usually solve the problem, but there may be times when it is necessary to tell the browser specifically what to do. This is a nuisance, and not everybody knows how to do it (the procedure has been made particularly opaque in the current version of Netscape, for some reason). In cases where the file is not being identified by the program or plug-in which would normally be expected to handle it, it is possible that the appropriate MIME type has not been set by the website administrator; in effect, the browser hasn't been told what the file is, and is therefore unable to decide what to do with it. Sometimes a browser will just guess that it must be a text file, and open it as such, which is rather a waste of time for all concerned. If anybody reports difficulties with media files on your site, the MIME type definitions are one of the first things you should check. How you do that will depend entirely on your webspace provider.

In this case, though, the most likely explanation is that your friend just hasn't got anything on his machine that will play .wav format. There are various programs for Mac that will do it, but as a non-user I can't help more than that. What I will say, though, is that .wav isn't a very good format for cross-platform internet use; you would probably do better to use mp3, for instance.