Available since LÖVE 0.10.0  
 This function is not supported in earlier versions. 
Gets the duration of the sound file. It may not always be sample-accurate, and it may return -1 if the duration cannot be determined at all.
duration = Decoder:getDuration( )
None.
number duration
    © 2006–2016 LÖVE Development Team
Licensed under the GNU Free Documentation License, Version 1.3.
    https://love2d.org/wiki/Decoder:getDuration