The main point I want to advocate is that when your audience is at a
distance, you will get much less nonverbal feedback, even if you use
audio. This is where a good presenter can shine by asking the leading
questions. With chat you can still do this, and you can reduce the
effects of accent and idiomatic differences. Also, chat requires much
less bandwidth, so now you can include a larger audience.
My experience is that requirements for live audio and live video usually
exclude about 40% of the potential audience. When I have tried live
audio, there is usually one participant that will require 45 minutes of
individual support to get their computer working.