NinjaCoder - The preview audio is provided by 7Digital. You need to agree to 7 Digital's TOS to use their assets. Read this for all of the details:
http://developer.echonest.com/sandbox/7digital.html
You need to add the 'limit=true' flag to your calls. This will limit results to songs that are in the 7digital catalog, which will always have the preview urls. Since the preview audio is maintained by 7Digital, their lifetime is set by 7Digital. Historically, they've been stable, but there's no guarantee.
-- Paul