This is the playlist, I do not know where I have to put the volume change, I tested and does not work, please help.
<iframe allowfullscreen="" frameborder="0" height="200" src="https://www.youtube.com/embed/videoseries?list=PLZtfdXg51ABw32MoMvZRQdN_EX9NHZLC8&autoplay=1" width="200"></iframe>
You probably can't do that, modifying an iframe doesn't work.
I recommend that you use the video tag. This gets a bit more complicated, but there are good answers for that: Show Youtube video source into HTML5 video tag?
When you have a video tag with the specific youtube video, you can then adjust the volume with javascript:
var video = document.getElementById("myVideo");
video .volume = 0.2; // 20% volume
