As far as I’m aware, 192kbps is regarded as the lowest bitrate for an mp3 to qualify as ‘high quality.’ That said, 128kbps is still good enough for most listeners, and I have done mixes at this rate that have gone down ok. Once you start coming down below that, you’re dealing with a considerable amount of loss through compression of the sound file. When I upload samples to illustrate a review, these are required to be 96kbps. They do the job of giving people an idea, but I wouldn’t want to listen to too much at this rate, especially when dealing with Ambient, where texture and sound colour are a major parameter. A podcast at 64kbps would be a pretty poor representation of such sounds, in my view.