Since oDesk does not guarantee payment for fixed price jobs, is anyone using a watermark for video or audio projects until payment is made? I've done that with other clients and it's worked well.
My preferred approach for my audio stuff (even outside of oDesk) is to send a non-downloadable private soundcloud link.
This works okay for most types of review, though soundcloud stream quality, while not bad, is not adequate for super high fidelity assessment.
In that case, then putting in a periodic beep as a watermark to a file works ok.
I should stress though, that I seldom worry about this stuff. Ultimately, there's not a lot you can do about a client who is hell-bent on ripping you off, other than to get good at spotting them in advance and avoiding them.
We do offer payment protection on Fixed Price jobs. Check out our Help documentation on using Escrow and Milestones to manage payments for these jobs. In addition to the best practices around watermark, etc. that you'll get from others here on the thread, we strongly recommend working with your client to manage payments with milestones and Escrow so you can get paid along the way.