Peachpit has a PDF with some iChat tips.
Relevant for us: if the audio quality degrades during the call, mute the audio and unmute it again: the connection will recalibrate and the sound will (may) improve.
Another setting that may prevent choking is ‘Bandwidth limit’. (This may ‘Lower iChat’s expectations’.)
For just Audio I could imagine the lowest setting would be appropriate but I have to test this.