I am also having the same problem as @jovica's.
I have tested on .raw, .wav, and .pcm files.
The files are me saying some valid sample utterance of my bot.
While the Lex console recognize what I am saying every time(so I think the issue is that my pronunciation.), the response from boto3 post_content, seems it doesn't know what I was saying.
(The wav file is me saying "go to the kitchen", however, the 'inputTranscript' returned is 'a a allen')
Can someone tell me what I've done wrong? Thanks.
Mine code is the following.

import boto3
client = boto3.client('lex-runtime')

f = open(WAVE_OUTPUT_FILENAME, 'rb')
lex_response = client.post_content(
botName = 'ProtoBot',
botAlias = 'ProtoBotFeb',
userId = "12345678910",
inputStream = f,
accept='text/plain; charset=utf-8',
contentType="audio/l16; rate=16000; channels=1"
print lex_response