I don't believe there is a way to do what you are looking for mostly because there isn't much of a practical reason for it. Pretty much any reasonably modern connection is going to handle streaming audio without buffering, so running multiple bitrates and trying to switch between them on the fly shouldn't be necessary. Also, changing from stereo to mono as bandwidth changes would likely have a negative impact on the overall listening experience.
Due to the extremely limited to non-existent need for such bandwidth scaling technology on audio only feeds, I'm not aware of any format or streaming server that supports this. That isn't to say there might not be one, but none of the major media servers I know of bother. 128Kbps is only a little over twice the speed of an old 56k modem. Even the most basic of DSL and cellular data connections exceed that.
Adaptive Bitrate Streaming is the term you are looking for if you want to do more digging to see if you can find something that does it for audio only that I have missed.