Audio Error : Can't play because data is NULL

I am getting warning in terminal when i run audio.play(filePath)
audio error : Can’t play because data is NULL

I am using 2012.2189.I also test on 2014.2100.But same error.
I am testing in mac simulator.

My code is:

local selectedWord = “food”

local selected_word_file = “sound/food.mp3”

local aa = audio.loadSound(selected_word_file)
audio.play(aa)

and get warning in terminal :-  
audio error : Can’t play because data is NULL

and audio file is not playing…

But when i run below code then works fine.

local aa = audio.loadSound(“sound/food.mp3”)
audio.play(aa)

and audio file play…

This is strange!!!

Can anybody help me why this is happening ?

Thanks in advance.