Odd line under "volume control"

That is caused by the entry-content a { border-bottom: 2px solid; } line of code in the theme’s CSS. That line of code says this: If there’s a link in a blog post, page post, or comment post, put a solid border at the bottom that’s 2px in width.

If you don’t want to appear on the volume control link, you can add this to your custom CSS area or child-theme’s CSS file to overwrite that:

.mejs-controls a.mejs-horizontal-volume-slider {
    border-bottom: none;
}

The code above says: If it’s a volume control link, don’t put any border at the bottom.

Extremely precise answer. Thank you for your support and for providing an explanation, more than just point me to some lines of code, really appreciated.