friendenstein/layouts/shortcodes/audio.html

<figure {{ with .Get "class" }}class="{{ . }}"{{ end }}>
  <audio loop controls preload="{{ .Get "preload" | default "metadata" }}">
  {{ with .Get "src" }}<source src="{{ .  | relURL }}" type="audio/mpeg">{{ end }}
  {{ with .Get "caption" }}<figcaption>{{ . }}</figcaption>{{ end }}
  </audio>
</figure>