You can test drive Google Audio Indexing or GAudi at Google Labs.
I have taken it for a quick spin a few minutes earlier and found that it can really find keywords in videos! Cool!
That was the good news and now for the bad news. Google Audio Indexing is brand new, so please do not expect that you can start searching for something in all videos right now. Google Labs is currently testing this new service and so far only videos in YouTube’s Politicians Channels have been indexed with Google Audio Indexing.
Check it out if you like!
Google Audio Indexing now on Google Labs