[Coroner]

You can try the following:
open umg_extensions.php

locate:

case 'flv':
		$width  = $ext['flv']['width'];
		$height = $ext['flv']['height'];
		$url    = $vbulletin->options['bburl'] . '/' . $media_url;
		$src    = 'umg_gallery/player/flvplayer.swf';
		$mtpl   = 'flashext';
		$mime   = 'application/x-shockwave-flash';
		break;

a.) if your code is the NOT the same, change it like this
b.) if your code IS the same like this, change:

$url    = $vbulletin->options['bburl'] . '/' . $media_url;

into

$url    = $media_url;