m4a audio format unable to reduce bitrate from 112k could this change be made?

hi there,
when i choose the m4a audio format I can't change the bit-rate from 112k to another one say 32 kbps or 64 kbps.
could it be made that the bit-rate, audio frequency could be changeable?
also unable to change the audio channels from 2 to one could this option be enabled?
thanks,
Majid Hussain