I found a solution (learning by doing wrong). ;-)

Code:
on !*:join:#testchannel: {
  if ($len($nick) >= 3) {
    var %x = 1
    while (%x <= $len($nick)) {
      .splay testchannel.wav
      .splay -q $+($mid($nick,%x,1),.wav)
      inc %x
    }
  }
}