There is no sense to talk about bitrate without discussing audio codecs. Some of them are made specifically for low bitrate broadcasts over the Internet. For example, I listen to a news radio station that uses Windows Media 20kbps audio stream; the broadcast is not perfect, but I can understand everything perfectly.

Why don't you just record your own voice to something like MP3, WMA, AAC, OGG using several bitrates and see what is good enough for you?

When you broadcast an audio file without changing bitrate, the bandwidth used is not much bigger than the size of the file. I haven't tried to estimate this, but if you add 10% to the file size it should give you a pretty good estimate.

The bandwidth of your server should probably allow to record several users simultaneously. How many users? It depends on you...