@JonB thank you for your prompt response. What you suggested works: A wav file gets created.
Unfortunately, AWS lex still does not seem to understand it once I submit it.
Is there a way to capture a MPEG audio? That works as I receive MPEG response from Lex bot which I have then submitted right back to it and it worked like charm, bot can "understand it and response shows that in inputTranscript text that gets returned.