Ah! So, I suggest using the following method:
Code:
; set file
var %file = $shortfn(somedir\something.mp3)

; set length in seconds ($sound(x).length returns milliseconds)
var %len = $calc($sound(%file).length / 1000)

; use the $duration identifier and the ,3 switch to get an output "hh:mm:ss"
var %dur = $duration(%len,3)

; use $gettok to grab the last 2 tokens "mm:ss", add these square brackets
return $+($chr(91),$gettok(%dur,2-3,58),$chr(93))


condensed to one line:
Code:
 return $+($chr(91),$gettok($duration($calc($sound($shortfn(somedir\something.mp3)).length /1000),3),2-3,58),$chr(93))