Thanks for noting that.
It's actually a parameter mistype, there is no need to hardcode your custom folder.
So in order to fix this you should replace:
// video
$vfolder = $pluginParams->def('video_folder','images/stories/videos');
$vwidth = $pluginParams->def('vwidth',400);
$vheight = $pluginParams->def('vheight',300);
// audio
$afolder = $pluginParams->def('audio_folder','images/stories/audio');
$awidth = $pluginParams->def('awidth',300);
$aheight = $pluginParams->def('aheight',20);
with this
// video
$vfolder = $pluginParams->def('vfolder','images/stories/videos');
$vwidth = $pluginParams->def('vwidth',400);
$vheight = $pluginParams->def('vheight',300);
// audio
$afolder = $pluginParams->def('afolder','images/stories/audio');
$awidth = $pluginParams->def('awidth',300);
$aheight = $pluginParams->def('aheight',20);
I'm releasing v2.5.3 tomorrow to fix this.
Agaian thanks to SuperStitch for noting. ;)