Edit the file inc\classes\BxDolSharedMedia.php
At about line 704 look for the following code.
function getBlockCode_SharedMedia($iUser = 0) {
$iUser = (int)$iUser;
$aManage = array('medID','medExt','medTitle','medUri');
$max_num = (int)getParam("top_photos_max_num");
$mode = process_db_input( getParam("top_photos_mode") );
$mode = $_GET['sh_'.$this->sType.'Mode'];
if( $mode != 'rand' && $mode != 'top' && $mode != 'last')
$mode = 'last';
$aCond['sqlWhere'] = " WHERE `{$this->aTableFields['Approved']}`='true'";
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Make it look like this by adding the line in red.
function getBlockCode_SharedMedia($iUser = 0) {
$iUser = (int)$iUser;
$aManage = array('medID','medExt','medTitle','medUri');
$max_num = (int)getParam("top_photos_max_num");
$mode = process_db_input( getParam("top_photos_mode") );
if ($_GET['sh_videoMode'] == '') $_GET['sh_videoMode']='rand';
$mode = $_GET['sh_'.$this->sType.'Mode'];
if( $mode != 'rand' && $mode != 'top' && $mode != 'last')
$mode = 'last';
$aCond['sqlWhere'] = " WHERE `{$this->aTableFields['Approved']}`='true'";
Save.
Video section should now default to random.