In order to convert your audio files to text documents, I suggest you to use an online utility named VoiceBase. It supports .aac, .aiff, .amr, .asf, .au, .avi, .caf, .cf, .flac, .flv, .mov, .mpeg, .mpg, .mp3, .mp4, .m4a, .m4v, .ogg, .ra, .wav, .webm, .wma, .wmv and .3gp files. Create a free VoiceBase account, click Upload audio button from the left, select your file click Start. You’ll be notified by e-mail when the process is finished. After that, go to My Uploads tab, select the file, check the Show Raw Machine Transcript box, copy the transcript file and save it as a text document.