How can a bot receive a voice file from Facebook Messenger (MP4) and convert it to a format that is recognized by speech engines like Bing or Google? - audio

Question

I'm trying to make a bot for Facebook Messenger using Microsoft's Bot Framework that will do this:
Get a user's voice message sent via Facebook Messenger
Convert speech to text
Do something with it
There's no problem with getting the voice message from Messenger (the URL can be extracted from the message the bot receives), and there's also no problem with converting an audio file to speech (using Bing Speech API or Google's similar API).
However, these APIs require PCM (WAV) files, while Facebook Messenger gives you an MP4 file.
Is there a popular/standard way of converting one format into another that is used in writing the bots?
So far my best idea is to run vlc.exe as a console job on my server and convert the file, but that doesn't sound like the best solution.

Answer 1

Developed a solution that works as follows:
Receive voice message from facebook
Download the MP4 file to local disk using the link inside Activity.Attachments
Use MediaToolKit (wrapper for FFMPEG) to convert MP4/AAC to WAV on local server
Send the WAV to Bing Speech API
So the answer to my question is: use MediaToolKit+ffmpeg to convert the file format.
Sample implementation and code here: https://github.com/J3QQ4/Facebook-Messenger-Voice-Message-Converter
public string ConvertMP4ToWAV()
{
var inputFile = new MediaFile { Filename = SourceFileNameAndPath };
var outputFile = new MediaFile { Filename = ConvertedFileNameAndPath };
using (var engine = new Engine(GetFFMPEGBinaryPath()))
{
engine.Convert(inputFile, outputFile);
}
return ConvertedFileNameAndPath;
}