The older Instreamer100 generates VBR (variable bitrate), so an exact bitrate cannot be configured, but if you use the right settings the bitrate will average +-5% to a value near to or below 64kbps (or the desired value).

With the new Instreamer you can select CBR bitrate 64kbps and you should select a lower audio frequency (e.g. 22.05 kHz).