A lot of my voice acting work is paid for once, within the same week, these days mostly through the shingle I hung on oDesk last October. But I've also been working with Silverton Audio for the last four months to record audiobooks, typically in exchange for future, ongoing royalty payments. The idea is that you never quite know what's going to sell and what won't, but statistically, over time, the approach should yield more total income than a single upfront payment.
I just got word that my efforts' first audiobook is now available for sale, (and apparently is already selling), on Amazon's Audible.com: Taming Tessa: Montana Maiden, Book 2 (NSFW) by Vanessa Vale. A second one is very close to launch: End Times: A Comedy by Bill and Bob Schmalfeldt. Apparently this also means that I get my own Narrator page on Audible. (Hey, I have my own IMDb profile, too, but at least Audible spelled my name right; heh.)
Note to self: I need to get set up with Amazon/Audible's affiliate program(s), so I can start getting credit for such links.