To show duration in minutes and seconds, edit /tmplates/index.tpl

FIND

Code:
Runtime: {$answers[i].duration|string_format:"%.2f"} |
REPLACE WITH

Code:
{assign var=viddur value=$answers[i].duration} 
{math equation="$viddur/60" format="%0.0f" assign=minutes} 
{math equation="$viddur - ($minutes * 60)" format="%0.0f" assign=seconds} 
{if $seconds < 0} 
{math equation="$minutes - 1" assign=minutes} 
{math equation="$seconds + 60" assign=seconds} 
{/if}            
Runtime: {$minutes}m{$seconds}s |
We will look into the duration problem, this may be due to ffmpeg-php is not able to get duration from video format you are uploading.