Sending speech by post request

Last week I asked a question about how can I use microphone via API. [1]

As per the answer, I can send base64 encoded audio like {"speech" : base64audio}

Although this hasn't worked for me. The bot always says "Sorry, could not recognize your message, please try again."

I am pretty sure I encoded my audio file to base64 string.
I tried sending a base 64 encoded audio from command line as well.
In both the cases the response was "Sorry, could not recognize your message, please try again.".

I am not sure where I went worng. Could I get some help on this?

[1] https://support.snatchbot.me/discuss/5efb5a3939ca8f004991a7da