madsemildr Hi. I see what happened. It was an editor which has turned the url to a link, so the code for plugin looked like:
{s5_media_player media=[audio] audio_mp3=[<a href="/images/audio/Vidder4.mp3">/images/audio/Vidder4.mp3</a>]}
instead of
{s5_media_player media=[audio] audio_mp3=[/images/audio/Vidder4.mp3]}
I had to correct it in HTML mode. Now it works.