In your UT2004 game folder, look at the Music subfolder, and open the file "APubWithNoBeer.ogg". What in the hell is this doing here? Is there some secret scene in the game that this goes with? Is it there as by itself as a joke?

Clicky ;)

I...see. Can I play this scenario with bots? Or is it strictly online multiplayer? Are there any other "joke" maps?

It's an assault map (i.e. objective based); you can play it with bots or online.
As regards "joke" maps, I think this is pretty much the only one.

I'm not sure if there are any other joke maps, can't think of one at least.
Oh, and 'A pub with no beer' thread simply must have Slim Dusty, so here he is. Enjoy ;)