Try
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
}

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