I recommend to use SoundFlower, the great audio routing program.

When using modules in AVFoundation, sound environment needs to be configured properly both input and output.
But MacPro2013 doesn't have internal microphone so it cannot be configured properly and when playing sound with AVAudioPlayer, errors like below will appear in console.

ruby>>
2014-04-09 11:50:47.717 XXXX[2897:60b] 11:50:47.717 ERROR: 398: error -66680
2014-04-09 11:50:47.717 XXXX[2897:60b] 11:50:47.717 ERROR: 398: error -66680
2014-04-09 11:50:47.717 XXXX[2897:60b] 11:50:47.717 ERROR: >aq> 1605: failed (-66680); will stop (11025/0 frames)

<<--

Because I don't have any microphones around me, I cannot plug it into my MacPro. I don't like to plug my earphone instead of plugging microphone.
There was another choice. Soundflower, the great software for routing audio. Installing this software and selecting SoundFlower(2ch) in Setting/Sound/Input solved this problem.