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I don't think it's appropriate to put Realaudio on Wikipedia. The only sound format that is readily usable by freely-available software on multiple platforms and that is compressed for Web transfer is Ogg Vorbis. Especially now that they've released 1.0, I see no reason to use anything else.

Can ordinary Windows users, i.e. users who aren't into music and file sharing and the like, use Ogg Vorbis with relative ease? For example, could my mom, running Windows 2000 or Windows XP, click on an Ogg Vorbis sound and end up happy?

She probably doesn't have the software installed already, but it's not that difficult. Some players like Sonique come with OGG support out-of-the-box; others like WinAmp require you to install a plugin. It's likely that Mozilla will add native support. I.E. will likely always launch the external player.

But the big benefit is that unlike RA, she won't have to install software that takes over her machine, sends her spam, and was written by a company that has proven it can't be trusted. As much as people complain about MS, Real.com is ten times worse. I personally refuse to use any software from them--I value my computer, and won't risk it. So I simply can't use RA at all. It's a one-company format, so if I don't use their software, there's no alternative.

I'm not quite as opposed to MP3--there are some patent problems, but at least it's not a one-company format and there are players for all platforms, and players are more likely to support it without pugins. But most software supporting MP3 is also supporting OGG these days or has promised to.