Change default FFmpeg audio codec to libvo-aacenc

After running more tests, AAC volume was not that low compared to
Vorbis:

https://dl.dropboxusercontent.com/u/84476328/ShareX/2014-05/2014-05-15_05-10-26.mp4

https://dl.dropboxusercontent.com/u/84476328/ShareX/2014-05/2014-05-15_05-10-03.mp4

https://dl.dropboxusercontent.com/u/84476328/ShareX/2014-05/2014-05-15_05-09-35.mp4