Update: I’ve been trying for an hour.
First I installed the WP plugin “Media Element.js” and tried it with [ audio src ] shortcode (in the visual tab) like they told on the plugin site. It worked in IE. In Firefox the player appeared for a short instant but then disappeared.
Then I realized that – obviously – the Media Element js stuff is already integrated in the Pinboard theme. On the demo site (link in opening post) I saw in the output source code that the player used HTML5 (less than/greater than) tags. I tried this on my blog (in the html tab), too. Result: In IE it works, but in Firefox I see only the player background (black/grey gradient) but no controls.
Can you help me please?