GSAP timeline to speedup / slowdown any animations on it

first problem - the button script in the slidemaster

wrong:

var allAudios = document.getElementsByTagName("audio");
for (var i = 0; i < allAudios.length; i++) {
   allAudios[i].playbackRate = 0.70;
}
...

correct:

var allAudios = document.getElementsByTagName("audio");
for (var i = 0; i < allAudios.length; i++) {
   var mp3URL = allAudios[i].src;
   var tmpArr = mp3URL.split("/");
   mp3Name = tmpArr[tmpArr.length-1];
   if(mp3Name != "FondoSusten.mp3"){
      allAudios[i].playbackRate = 0.70;
   }
}
...

second problem: audio is working correct after slider change - your script is correct

BUT: you have to update the navigation button also (on timeline start on every slide)