Code:
very basic player

dialog MP3play {
title "MP3 Player"
size -1 -1 200 100
option dbu
list 100, 5 5 190 60
button "PLAY", 900, 75 75 50 25
}

on *:dialog:mp3play:init:0:{
var %null = $findfile($mp3dir,*.mp3,0,did -a mp3play 100 $nopath($1-))
}
on *:dialog:mp3play:sclick:900:{
splay $did(mp3play,100).seltext
}

more basic 
alias mp3 splay $sfile($mp3dir*.mp3)