Sound.duration
Get the sound is length in seconds
Example
--! luart-extensions
import audio
local sound = audio.Sound("music.mp3")
local time = sys.Datetime()
-- Calculate the length of the audio
time.hour = 0
time.minute = math.floor(sound.duration/60)
time.second = math.floor(sound.duration - time.minute*60)
print("music.mp3 length is "..time.time)