Hm, thew few webinars I've been in had both an awkward login-procedure quite hard to automate, and didn;t have sound option, but for a (toll-free) dial-in number that I had to listen to from my mobile phone (like the rest of the company, having no wired telephone on my desk...)
Any web-capture software that allows/features to be scheduled, should be able to handle this task though, after you've skipped the initial rough-spots I mentioned above.