Here's a really simple example, shows you the basics. Change file.txt to the real filename and type "/xmp3x"
Code:
alias xmp3x dialog -m xmp3x xmp3x
dialog xMP3x {
  title mp3
  option dbu
  size -1 -1 200 50
  combo 1, 5 5 190 100,size drop hsbar vsbar
  button "Play" 2, 50 26 40 15
  button "OK" 3, 100 26 40 15,ok
}
on *:DIALOG:xMP3x:init:0:{
  filter -fo [color:blue]file.txt[/color] xMP3x 1 *.mp3
  did -z xMP3x 1
}
on *:DIALOG:xMP3x:sclick:2:{
  splay $$did(1)
}