Why can't I play audio, and how can I fix it?

Question

css

codepen.io/anon/pen/mejVqw There is a button that should play audio, but it is not clickable. How to fix it?

Answer 1

.frag
{
    z-index: 500;
}

add position: relative; here, z-index: doesn't work without position: relative, absolute, fixed;