Specifying Duration
You can use the dur attribute to specify the amount of time that a media element can be played. This is illustrated in the following example. The first and second elements play for 30 seconds or their actual length, whichever is less. Because a Windows Media server does not recognize negative durations, the third element plays for its actual length.
<?wsx version="1.0"?>
<smil>
<seq>
<ref src="c:\wmpub\wmroot\audio1.wma" dur="+.5min" />
<ref src="c:\wmpub\wmroot\audio2.wma" dur="0.5min" />
<ref src="c:\wmpub\wmroot\audio3.wma" dur="-55" />
</seq>
</smil>