After a bit of research and help from other members, I got what I was looking for. This answer is for those who may land here searching for bitrate property of audio files.
First of all, if we use Shell then bitrate property will always be found at index 28. However it is upto Shell object if it contains any value for that property. as Shell's main purpose is not to read the audio files, so we should not be relying on it to read any audio file properties.
This Thread explains what we need to do to read the .wav's bitrate.