IMO, broadcast TV for soccer games has had the right idea for advertising that can be co-opted for online video.

Soccer game broadcasts usually play two 45 minute halves with very few breaks for commercials. So, they do a lot of banner advertising along the bottom of the screen. Sometimes it's a text crawl, sometimes it's animation. I've noticed that Google has been experimenting with this on YouTube recently.

As to context sensitive advertising... that's a tough one. Probably your best bet would be tagging. But, if you know that most of your audio is going to be spoken word into a microphone, that makes your job a bit simpler. You could try running a speech recognition program on your audio stream, and then pick up repeated words from that to use as contextual ads.