http://c2.com/cgi/wiki?ThinkingMusicConsideredHarmful

My personal favorite is

$ dd if=/dev/random of=/dev/audio      # or /dev/dsp on Linux

Of course, if there are other users on your system, this will get them rather torked off at you if they need to use pgp or ssh.

And if there aren't other users, /dev/random will probably produce its output slowly enough that you won't actually hear anything. This is the case, at least, under Linux 2.4, where /dev/random is relatively random. However, /dev/urandom works fine.