I've spoken to a dude named Maurice LaMarche in the past, he's a pretty established voice over actor - practically a legend - and he recommended to do whatever the agent says, because good agent will tell you what you need to do to appeal to their clientel. HOWEVER... if you don't have one, and you're representing yourself (which means you won't even get into the screening process for most auditions without some type of real representation), three minutes of a well-written, multi-voiced comedy sketch will give you an advantage over other talent. Three minutes goes fast when it's good... but unfortunately, they discourage most from doing this since the material sucks and most voice-over actors aren't professional-level writers - and, now that I think of it, you really wouldn't have more than one good reference to work with.
I just assumed that it would work for a mixed demo... but I can see your logic of it being too much to digest since there's no actually storyline, just sound clips.
Either way, freelance *anything* is a bitch and I commend anyone who can work hard enough to keep themselves fed and make a living at it. I quit animation and comic book illustration because it was a very "who knows who" industry and extremely elitist.