Question: I would like to realize this.  A user speaks to a web browser. A web browser (Google Chrome) record user's voice as WAV file(Recorder.js) and send it to a python-flask server.  I realized this with the help of addpipe's simple recorder.js sample. https://github.com/addpipe/simple-recorderjs-demo This sample uses php server, so I changed the original app.js. original app.js xhr.open("POST","uplod.php",true);  my app.js xhr.open("POST","/",true);  I checked my web app locally, then everything looked perfect. I use Windows 10, WSL, Debian 10/buster, python3.7.6, Google Chrome. Here is Terminal's record. 127.0.0.1 - - [03/Feb/2020 11:53:17] "GET / HTTP/1.1" 200 - ./file.wav exists 127.0.0.1 - - [03/Feb/2020 11:53:32] "POST / HTTP/1.1" 200 -  However, when I checked uploaded "file.wav" with ffprobe command, it was broken.  libavutil 56. 22.100 / 56. 22.100 libavcodec 58. 35.100 / 58. 35.100 libavformat 58. 20.100 / 58. 20.100 libavdevice 58. 5.100 / 58. 5.100 libavfilter 7. 40.101 / 7. 40.101 libavresample 4. 0. 0 / 4. 0. 0 libswscale 5. 3.100 / 5. 3.100 libswresample 3. 3.100 / 3. 3.100 libpostproc 55. 3.100 / 55. 3.100 file.wav: Invalid data found when processing input  This is screenshot of my app. When I push "save to disk" button, I can download WAV file locally. If I check downloaded WAV file, it is not broken.  libavutil 56. 22.100 / 56. 22.100 libavcodec 58. 35.100 / 58. 35.100 libavformat 58. 20.100 / 58. 20.100 libavdevice 58. 5.100 / 58. 5.100 libavfilter 7. 40.101 / 7. 40.101 libavresample 4. 0. 0 / 4. 0. 0 libswscale 5. 3.100 / 5. 3.100 libswresample 3. 3.100 / 3. 3.100 libpostproc 55. 3.100 / 55. 3.100 Input #0, wav, from '/mnt/c/Users/w0obe/Downloads/2020-02-03T02_53_29.366Z.wav': Duration: 00:00:07.34, bitrate: 768 kb/s Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, 1 channels, s16, 768 kb/s    My directory structure is here. . ├── file.wav(uploaded WAV file) ├── main.py ├── Pipfile ├── Pipfile.lock ├── static │ └── js │ └── app.js └── templates └── index.html  This is main.py. #!/usr/bin/env python # -*- coding: utf-8 -*- from flask import Flask from flask import request from flask import render_template import os app = Flask(__name__) @app.route("/", methods=['POST', 'GET']) def index(): if request.method == "POST": f = open('./file.wav', 'wb') f.write(request.get_data("audio_data")) f.close() if os.path.isfile('./file.wav'): print("./file.wav exists") return render_template('index.html', request="POST") else: return render_template("index.html") if __name__ == "__main__": app.run()  This is index.html. <!DOCTYPE html> <html> <head> <meta charset="UTF-8"> <title>Simple Recorder.js demo with record, stop and pause - addpipe.com</title> <meta name="viewport" content="width=device-width, initial-scale=1.0"> </head> <body> <h1>Simple Recorder.js demo</h1> <div id="controls"> <button id="recordButton">Record</button> <button id="pauseButton" disabled>Pause</button> <button id="stopButton" disabled>Stop</button> </div> <div id="formats">Format: start recording to see sample rate</div> <p><strong>Recordings:</strong></p> <ol id="recordingsList"></ol> <!-- inserting these scripts at the end to be able to use all the elements in the DOM --> <script src="https://cdn.rawgit.com/mattdiamond/Recorderjs/08e7abd9/dist/recorder.js"></script> <script src="/static/js/app.js"></script> </body> </html>  This is app.js. //webkitURL is deprecated but nevertheless URL = window.URL || window.webkitURL; var gumStream; //stream from getUserMedia() var rec; //Recorder.js object var input; //MediaStreamAudioSourceNode we'll be recording // shim for AudioContext when it's not avb. var AudioContext = window.AudioContext || window.webkitAudioContext; var audioContext //audio context to help us record var recordButton = document.getElementById("recordButton"); var stopButton = document.getElementById("stopButton"); var pauseButton = document.getElementById("pauseButton"); //add events to those 2 buttons recordButton.addEventListener("click", startRecording); stopButton.addEventListener("click", stopRecording); pauseButton.addEventListener("click", pauseRecording); function startRecording() { console.log("recordButton clicked"); /* Simple constraints object, for more advanced audio features see https://addpipe.com/blog/audio-constraints-getusermedia/ */ var constraints = { audio: true, video:false } /* Disable the record button until we get a success or fail from getUserMedia() */ recordButton.disabled = true; stopButton.disabled = false; pauseButton.disabled = false /* We're using the standard promise based getUserMedia() https://developer.mozilla.org/en-US/docs/Web/API/MediaDevices/getUserMedia */ navigator.mediaDevices.getUserMedia(constraints).then(function(stream) { console.log("getUserMedia() success, stream created, initializing Recorder.js ..."); /* create an audio context after getUserMedia is called sampleRate might change after getUserMedia is called, like it does on macOS when recording through AirPods the sampleRate defaults to the one set in your OS for your playback device */ audioContext = new AudioContext(); //update the format document.getElementById("formats").innerHTML="Format: 1 channel pcm @ "+audioContext.sampleRate/1000+"kHz" /* assign to gumStream for later use */ gumStream = stream; /* use the stream */ input = audioContext.createMediaStreamSource(stream); /* Create the Recorder object and configure to record mono sound (1 channel) Recording 2 channels will double the file size */ rec = new Recorder(input,{numChannels:1}) //start the recording process rec.record() console.log("Recording started"); }).catch(function(err) { //enable the record button if getUserMedia() fails recordButton.disabled = false; stopButton.disabled = true; pauseButton.disabled = true }); } function pauseRecording(){ console.log("pauseButton clicked rec.recording=",rec.recording ); if (rec.recording){ //pause rec.stop(); pauseButton.innerHTML="Resume"; }else{ //resume rec.record() pauseButton.innerHTML="Pause"; } } function stopRecording() { console.log("stopButton clicked"); //disable the stop button, enable the record too allow for new recordings stopButton.disabled = true; recordButton.disabled = false; pauseButton.disabled = true; //reset button just in case the recording is stopped while paused pauseButton.innerHTML="Pause"; //tell the recorder to stop the recording rec.stop(); //stop microphone access gumStream.getAudioTracks()[0].stop(); //create the wav blob and pass it on to createDownloadLink rec.exportWAV(createDownloadLink); } function createDownloadLink(blob) { var url = URL.createObjectURL(blob); var au = document.createElement('audio'); var li = document.createElement('li'); var link = document.createElement('a'); //name of .wav file to use during upload and download (without extendion) var filename = new Date().toISOString(); //add controls to the <audio> element au.controls = true; au.src = url; //save to disk link link.href = url; link.download = filename+".wav"; //download forces the browser to donwload the file using the filename link.innerHTML = "Save to disk"; //add the new audio element to li li.appendChild(au); //add the filename to the li li.appendChild(document.createTextNode(filename+".wav ")) //add the save to disk link to li li.appendChild(link); //upload link var upload = document.createElement('a'); upload.href="#"; upload.innerHTML = "Upload"; upload.addEventListener("click", function(event){ var xhr=new XMLHttpRequest(); xhr.onload=function(e) { if(this.readyState === 4) { console.log("Server returned: ",e.target.responseText); } }; var fd=new FormData(); fd.append("audio_data",blob, filename); xhr.open("POST","/",true); xhr.send(fd); }) li.appendChild(document.createTextNode (" "))//add a space in between li.appendChild(upload)//add the upload link to li //add the li element to the ol recordingsList.appendChild(li); }  How can I upload WAV file to flask server? Could you give me any information or suggestion? Thank you in advance. Sincerely, Kazu

Answer: I have this problem and it takes me 2 days for finding the solution :)) . In flask server you can use request.files['audio_data'] to get wav audio file. You can pass and use it as an audio variable too. Hope this can help you