Sometimes when iChat AV lacks enough bandwidth for high quality audio chat, it chokes the throughput by degrading audio quality. This can be a pain, as when you regain your full bandwidth, you'll still have poor audio quality conferencing because of a bottleneck in the past.

To let iChat re-evaluate its quality setting implicated by the available bandwidth, just push the mute button and then press it again to toggle the audio on and off -- voila!