Code:
dialog Mp3_dialog {  
  size -1 -1 298 415  
  title "Andys Mp3s"  
  list 1, 21 21 252 234,vsbar size   
  button "Play",2,31 314 60 30  
  button "Stop",3,112 314 60 30  
  box "Mp3 Dir",4,31 268 235 35  
  button "Change Dir",5,191 315 60 30
  text "", 6, 36 282 191 11
}

on *:dialog:Mp3_dialog:init:*: { 
 $findfile(%mp3dir,*.mp3,0,did -a Mp3_dialog 1 $nopath($1-)) 
 did -a Mp3_dialog 6 %mp3dir
}

Try that.