Try to extract the date from filenames by guessing with different formats.

grab_date.php

<?php
/**
 * Run over different date patterns 
 * and attempt to extract the correct date from a given string.
 *
 * @param  String containing date.
 * @return date in the YYYY-MM-DD format, or FALSE if no match was found.
 **/
function grab_date($str)
{
	$months_short = array('JAN', 'FEB', 'MAR', 'APR', 'MAY', 'JUN', 'JUL', 'AUG', 'SEP', 'OCT', 'NOV', 'DEC');
	$months_long = array('JANUARY', 'FEBRUARY', 'MARCH', 'APRIL', 'MAY', 'JUNE', 'JULY', 'AUGUST', 'SEPTEMBER', 'OCTOBER', 'NOVEMBER', 'DECEMBER');

	$patterns = array(
					'/([0-9]{2})[\-\.\_]([0-9]{2})[\-\.\_]([0-9]{4})/' => array(3,2,1),
					'/([0-9]{4})[\-\.\_]([0-9]{2})[\-\.\_]([0-9]{2})/' => array(1,2,3),
					'/([0-9]{1,2})[\-\.\_ ]?([a-zA-Z]{3,9})[\-\.\_ ]?([0-9]{4})/' => array(3,2,1)
				);
	
	foreach($patterns as $pattern => $indices)
	{
		if(preg_match($pattern, $str, $matches))
		{
			$parts = array();
			foreach($indices as $index) {
				$parts[] = $matches[$index];
			}
			
			// Deal with non-numeric months
			if(!is_numeric($parts[1])) 
			{
				$haystack = strlen($parts[1]) == 3 ? $months_short : $months_long;
				
				$search = array_search(strtoupper($parts[1]), $haystack);
				if($search !== false) {
					$parts[1] = $search + 1;
				}		
			}
			
			// Zero-pad month/day
			for($i=1;$i<3;$i++) {
				$parts[$i] = sprintf("%02d", $parts[$i]);
			}
			
			return implode('-', $parts);
		}
	}
	
	return false;
}

// Example usage: 
$filenames = explode("\n", file_get_contents('example.txt'));
foreach($filenames as $filename)
{
	if($filename == '') {
		continue;
	}

	$date = grab_date($filename);
	
	if($date !== false) {
		echo $filename . ' contains date: ' . $date . PHP_EOL;
    } else {
    	echo 'Could not find a date in ' . $filename . PHP_EOL;
    }
}

grab_date.txt

01-Markus_Schulz_-_Global_DJ_Broadcast_(DI.fm)-NET-2008-04-10-PS.mp3
01-Markus_Schulz_-_Global_DJ_Broadcast_(DI.fm)-NET-2008-08-28-PS.mp3
01-Markus_Schulz_-_Global_DJ_Broadcast_(DI.fm)-NET-22-02-2007-PS.mp3
01-Markus_Schulz_-_Global_DJ_Broadcast_-_ISS_-_guest_Jody_Wisternoff-NET-11-09-2008.mp3
01-markus_schulz-global_dj_broadcast_(the_mix)_(party_931)-cable-01-10-2005-mim.mp3
01-markus_schulz-global_dj_broadcast_(winter_music_conference)_(party_931)-cable-03-10-2004-mim.mp3
01-markus_schulz_-_global_dj_broadcast__world_tour_san_francisco-sbd-03-04-2010.mp3
01_markus_schulz_-_gdjb_ibiza_summer_sessions__incl_jody_wisternoff_guestmix-07-26-2007.mp3
01_markus_schulz_-_global_dj_broadcast_2006-11-30_(di.dm)-tt.mp3
Markus Schulz - Global DJ Broadcast (22-05-2008).mp3
Markus Schulz presents - Global DJ Broadcast  2010.04.08.mp3
Markus Schulz presents - Global DJ Broadcast World Tour (7 August 2008) - DJM.mp3
Markus_Schulz-Global_DJ_Broadcast_(2010-02-04).mp3
Markus_Schulz-Global_DJ_Broadcast_Cosmic_Gate_Guestmix-21-12-2006-TRANCEZONE.mp3
Markus_Schulz_-_Global_DJ_Broadcast_(10October2007)_URTranceZone.com.mp3
Markus_Schulz_-_Global_DJ_Broadcast_-_Guest_Matt_Darey-NET-26-06-2008.mp3
Markus_Schulz_-_Global_DJ_Broadcast__24JAN2008__URTranceZone.com.mp3
Markus_Schulz_-_Global_DJ_Broadcast__28FEB2008__URTranceZone.com.mp3
Markus_Schulz___Global_DJ_Broadcast__13_December_2007_www.trancezone.tv.pl.mp3
Markus_Schulz_presents_-_Global_DJ_Broadcast_(18_December_2008).mp3
Markus_Schulz_presents_-_Global_DJ_Broadcast_(18_September_2008).mp3
Markus_Schulz_presents_-_Global_DJ_Broadcast_(27_September_2007) HOUR I uploaded by BV.mp3
Markus_Schulz_presents_-_Global_DJ_Broadcast_(28_May_2009).mp3
Markus_Schulz_presents_-_Global_DJ_Broadcast_-_06.05.2010.mp3
Markus_Schulz_presents_-_Global_DJ_Broadcast_Ibiza_Summer_Sessions_(10_September_2009).mp3
Markus_Schulz_presents_-_Global_DJ_Broadcast__23_April_2009_.mp3