FlaskでRedirect（url_for('index')) の呼び出しが正常に動作しない

前提・実現したいこと

Flaskで音楽ファイルを再生するシステムを作っています。
音楽再生開始後にPLAYボタンを無効にし、
再生完了後にPLAYボタンを有効にする機能を実装中に
以下の問題が発生しました。

目的は、「再生完了時にPLAYボタンが再び有効にしたい。」

発生している問題・再生完了のcallbackメッセージまで問題ないですが、

下記の該当ソースでredirect部分が正常に動作しません。（呼び出されない）

^C(venv) pi@raspberrypi:~/hello_flask/sound $ python fl_play_all_format.py
 * Serving Flask app "fl_play_all_format" (lazy loading)
 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: on
 * Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 237-014-604
[2020-07-21 22:34:22,521] INFO in fl_play_all_format: 1
192.168.0.108 - - [21/Jul/2020 22:34:22] "GET /1 HTTP/1.1" 200 -
192.168.0.108 - - [21/Jul/2020 22:34:28] "POST /showfile HTTP/1.1" 200 -
Finish
[2020-07-21 22:34:44,024] INFO in fl_play_all_format: callback

該当のソースコード

Python
1ソースコード
2def wrap_play(filename):
3    snd = mpxplayer(filename)
4    re = snd.run()
5    app.logger.info("callback")
6    return redirect(url_for("index", val=1), code=307) #ここが正常に動作しない
7

試したこと（Pythonソース全体）

from flask import Flask, render_template, request, redirect, url_for
from MPXPLAYER import mpxplayer
from multiprocessing import Process
import glob

app = Flask(__name__)

def getmusicInfo():
    list = {}
    cnt = 0
    for file in glob.glob("/mnt/samba/*.mp*"):
        list[cnt] = file
        cnt = cnt + 1
    return list

@app.route("/<val>")
def index(val):
    app.logger.info(val)
    val = int(val)
    return render_template("mpindex.html", flag = val, selectedval = 0,
                            musiclink=getmusicInfo())

def wrap_play(filename):
    snd = mpxplayer(filename)
    re = snd.run()
    app.logger.info("callback")
    return redirect(url_for("index", val=1), code=307)

@app.route("/showfile", methods=['GET','POST'])
def main():

    wavFile= 0

    global play
    if request.method == 'POST':
        if request.form['bt'] =='PLAY':
            wavFile = str(request.form["musicfile"])
            musicFile = getmusicInfo()[int(wavFile)]
            play = Process(name="play", target=wrap_play, args=(musicFile, ))
            play.start()
            return render_template("mpindex.html", flag = 0, selectedval = int(wavFile),
                            musiclink=getmusicInfo())

if __name__ == "__main__":
    app.run(debug=True, host='0.0.0.0')

補足情報（HTMLソース）

HTML
1<!DOCTYPE html>
2<html lang="ja">
3  <head>
4    <meta charset="utf-8">
5    <meta http-equiv="X-UA-Compatible" content="IE=edge">
6    <meta name="viewport" content="width=device-width, initial-scale=1">
7    <link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-1q8mTJOASx8j1Au+a5WDVnPi2lkFfwwEAa8hDDdjZlpLegxhjVME1fgjWPGmkzs7" crossorigin="anonymous">
8    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js" integrity="sha384-0mSbJDEHialfmuBBQP6A4Qrprq5OVfW37PRR3j5ELqxss1yVqOtnepnHVP9aJ7xS" crossorigin="anonymous"></script>
9  </head>
10  <body>
11    <div class="container">
12      <div class="header">
13        <h3 class="text-muted">Music Page</h3>
14        <form action = '/showfile' method='POST'>
15          <select name="musicfile" style="font-size:16px;" id="listbox">
16          {% for key, value in musiclink.items() %}
17            {% if selectedval == key %}
18               <option value = "{{key}}" selected>{{value}}</option>
19            {% else %}
20               <option value = "{{key}}" > {{value}}</option>
21            {% endif %}
22           {% endfor %}
23           </select>
24           {% if flag == 1 %}
25                <input type="submit" name="bt" value="PLAY" id="bt1"></input>
26           {% else %}
27                <input type="submit" name="bt" value="PLAY" id="bt1" disabled></input>
28           {% endif %}
29        </form>
30<script type="text/javascript" language="javascript">
31         var select = document.getElementById( 'listbox' );
32          select.onchange = function(){
33          document.getElementById("bt1").disabled = false;
34          }
35</sciprt>
36      </div>
37    </div>
38  </body>
39</html>