Minimum bitrate below 500

I would like to be able to choose a bitrate that is lower than 500.

I am using RTMP server to send audio with a still image as video.

The video is irrelevent and the audio is the only thing that is important.

A bit rate of 500 is much higher than what is needed to get decent audio, so would like the be able to choose a lower rate like 160.

Thanks