What is the optimum bitrate to record human voice in normal audible quality

i am using a voice recorder to let users record their own voice. Now i want to know what is the bit-rate to record human voice in normal audible quality. I am not looking for very high quality. Recording will be done only through microphone or in built voice recorders in laptops. also what will be the streaming rate to record the voice. does it depend on users internet speed or my server speed. How to calculate the bandwidth?

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...